What size wrench do i need to loosen the caliper bolts on the front disc brakes?

Allen wrenches usually come in a set, try 3/8.

Instructions
1.Set the vehicle transmission in "Park" and engage the emergency brake before attempting to remove the front right axle. Place wood blocks behind the rear wheels to prevent the vehicle from rolling and loosen the lug nuts one complete turn on the right front wheel with a tire tool.

2. .Lift the front passenger side of the vehicle with a floor jack and slide a jack stand under the vehicle frame near the wheel. Lower the floor jack so the vehicle is sitting on the jack stand and move the floor jack out of the way.

3. Remove the lug nuts with the tire tool and set the front passenger wheel out of the way. Take the cap off the end of the axle with a standard screwdriver and remove the cotter pin from the axle with a pair of pliers.

4. Slide the crown washer off the axle and loosen the axle nut with a crescent wrench. Unfasten the top and bottom bolts that secure the brake caliper to the rotor with a socket wrench and slide the bolts out of the caliper.

5. Lift the brake caliper off the rotor and remove the brake pads from the caliper. Suspend the brake caliper from the vehicle frame with a wire hanger and take the rotor off the axle.

6. Unfasten the thru-bolt on the vertical sway bar with a socket wrench and remove the bolt from the sway bar. Take the channel bolt out from between the sway bar and lower A-arm that the thru-bolt ran through, and loosen the bolts that secure the front right axle to the center flange with a socket wrench and breaker bar. You may need to loosen the bolts with the breaker bar before removing them with the socket wrench.

7. Slide the front right axle off the center flange and let it rest on the A-arm. Lift the front passenger side up with the floor jack to get enough room between the steering knuckle and A-arm and slide the axle out of the vehicle.
Install in reverse order.

The caliper guide pins will usually have either a 3/8 inch or a 5/16 inch allen head bolt. If the brake calipers are found to have phenolic pistons then it would be advisable to change them with calipers that have metal pistons.

common hand tools and a torque wrench

Rear disc brake pads offer better performance and are not as affected by moisture like conventional brake shoe style brakes are. Rear disc brakes are similar to front disc brakes. The main difference is that rear disc brake systems must incorporate the emergency brake system. There are two methods widely used for the emergency brake with rear disc systems. The first system is a brake shoe inside the brake disc that is actuated by the emergency brake lever. The second is a screw style actuator inside the brake caliper. When activated the brake pads are forced into the brake disc and held tightly by the emergency brake lever.
READ COMPLETELY BEFORE STARTING
Step 1 - Identify Rear Disc Brake Components
Rear disc brake assembly includes; rear brake disc, rear brake pads, brake caliper mount and a caliper mounting screw. (Note: Some vehicles do not have the rotor mounting screw.)
Step 2 - Removing the Rear Brake Caliper Mount Bolts
To replace rear brake pads and rotors the rear brake caliper needs to be removed. First loosen the rear brake caliper mount bolts and remove them. Turn counter clockwise.
Step 3 - Lift Rear Brake Caliper from The Caliper Mount
After the caliper mount bolts have been removed, gently lift the brake caliper from the caliper mount. Inspect the caliper slides; they should move freely in the caliper mount. Remove rear brake pads and hardware.

Step 4 - Removing Caliper Mount Bolts
With a socket wrench or other appropriate removal tool, loosen the rear brake caliper mounting bolts. Remove bolts and lift the caliper mount and remove it from the vehicle. Remove the retaining screw from the disc mounting hole. Tap the rotor gently to release any rust that has accumulated between the rotor and bearing hub. Lift brake rotor from wheel hub holding on tightly, using both hands. You do not want to drop the rotor.

Step 5 - Removing Rear Brake Rotor
Remove the retaining screw from the disc mounting hole, tap the rotor gently to release any rust that has accumulated between the rotor and bearing hub. Lift brake rotor from wheel hub, hold on using both hands and do not drop.

Step 6 - Install New Brake Rotor
Check the new rotor against the old brake rotor to make sure they are the same size. Clean the mating surface on the wheel hub before the new brake rotor is installed. Reinstall rotor retainer screw.
Step 7 - Reset Rear Brake Caliper
Before new brake pads can be installed, the rear brake caliper must be reset. The reset tool winds the piston back into position so the new brake pads will fit. This style of brake caliper will not compress with a clamp tool; it can only be reset with the proper reset tool.
Step 8 - Reinstall Rear Caliper Mount and Install New Rear Brake Pads
After the caliper has been reset, reinstall caliper mounting bolts and make sure the bolts are tight. Then match up the old brake pads to the new brake pads. They should be exactly the same except, of course; the old ones will be worn out. Check the new brake pads for proper fit and install any brake hardware that is required.
Step 9 - Remount Rear Brake Caliper
Reinstall the brake caliper, align brake pad hardware and reinstall caliper mounting bolts. (Note: align the rear peg of the brake pad to the groove in the caliper piston.) Recheck and retighten all caliper and caliper mount bolts. Bleed brake system to relieve any air in the system. Before driving the vehicle, push the brake pedal down and let it up slowly. This operation forces the brake pads to travel to the brake rotors. DO NOT DRIVE VEHICLE until proper brake pedal operation resumes. When test driving vehicle listen for any unusual noises during the operation of the brakes.
WARNING! Always have the vehicle under inspection on level ground, in park with the emergency brake on. Always wear protective eyewear, gloves and necessary clothing before inspection or work begins. Never crank an engine over when anyone is near the battery or engine. Always have an operational fire extinguisher close by, obey all first aid instructions in the event of an injury. Never stand in front or behind a vehicle when cranked over or running. When engine is cranked over keep hands and clothing away from rotating components. Never move a car without proper brake pedal operation.

remove tire,loosen and remove caliper 2 bolts on rear side of caliper,usually a allen/torque wrench or a standard wrench if u got all 3 ur good,fully remove caliper disc slides right off studs,new caliper will be thicker,which requires the piston behind the pad to be moved in, u can use a large c lamp be very careful while tightening down on piston not to brake it usually lay something across the piston put the clamp on that piston must go in far enough to allow pad to slide over new disc good luck its easy and pretty self explanitory

A hex wrench of the proper size would do.

REMOVAL & INSTALLATION Front Brakes

1. Raise and safely support the vehicle.
2. Remove the wheel and tire assembly.
3. Unfasten the disc brake caliper retaining bolts.
4. Remove the disc brake caliper and secure the caliper aside with mechanic's wire. Do not disconnect the brake hose.
5. Pull the brake rotor from the hub.
6. Inspect the rotor and refinish or replace, as necessary. If refinishing, the minimum thickness rotor thickness is stamped on the brake rotor.
7. To install:
   
8. Place the brake rotor to the hub. Make sure the mating surfaces are free of rust which may prevent the brake rotor from seating properly.
9. Remove the mechanic's wire and position the brake caliper.
10. Install the brake caliper retaining bolts to specification.
11. Install the wheel and tire assembly.
12. Lower the vehicle.
13. Pump the brake pedal several times to position the brake pads before moving the vehicle.
14. Road test the vehicle and check the brake system for proper operation.

It sounds like your hub/rotor is froze on....you'll need to apply some penetrant in the lug stud areas as wells as the area where the rotor mates to the axle hub.

You will need a 7mm allen wrench to remove the caliper and the bolts come in from the back side once the two bolts are removed and you slide the caliper off you can use a c-clamp to push the cylinder back in to the caliper. Do not unhook any lines or loosen them and you will not need to bleed the brakes afterwards

it is a 3/8 allen wrench