Need access to service menu to adjust vertical size

To do this adjustment, you should enter the service mode option. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" will make surf easy.
http://electronicshelponline.blogspot.com/
Service mode entry, adjust and exit procedure to many popular brand TVs are given.

Zenith sets, like most others now, are adjusted electronically through a service menu. You'll need your original remote control (or a Zenith-brand universal replacement with a MENU key) to access the menu. Press and hold the MENU button until the channel number and time display goes away, then press 9, 8 , 7, 6, ENTER. This puts you into service mode.

In service mode, CHANNEL UP and CHANNEL DOWN move through the different settings, and VOLUME UP and VOLUME DOWN change the values. Make sure you write down the original value of anything before you change it so you can put it back if necessary. One of the reasons the manufacturers hide the service menu is because the set can be damaged by incorrect settings to some of the parameters.

On some models, you can't get to all the parameters unless you change one called F MODE from 00 to 01. You might need to do this on yours.

The list of parameters varies by model and features. I haven't worked on a Zenith in a long time so I don't remember exactly what yours says, but you want to find two settings for V SIZE (vertical size) and V POS (vertical position). Adjust the size first by lowering the value until the top and bottom of the screen are black. Then adjust the position so the picture is centered (black strips close to equal size). Finally, increase the size until the picture fills the screen again. It's normal to adjust for a little overscan, meaning you push the top and bottom a little bit out of view. If you stop too soon, on some channels you might see flashing lines at the top of the picture.

When you're finished adjusting, turn the TV off to exit service mode. If you needed to change the F MODE setting, put it back to 00 first. Your picture should be properly sized after all this.

You could go into the user menu and look around to see if it has an adjustment. If not you will have to access the service menu for your tv. There may be a setting to adjust vertical size there. If that still isn't possible you may have to open the tv and look for a pot or a switch that can control the vertical adjustment. It may be by the vertical output circuitry. This could also mean that you have a defective component in the vertical circuit or inadequate voltage to that circuit.

Hello
There are Horizontal and Vertical size controls behind the back cover that need to be adjusted using a hand held tool. This adjustment requires a trained tech. High Voltage from the set is dangerous.
However, there maybe some limited controls that may help through the on screen menu.
To get into the setup menu, you need the original Remote that came with the TV.
Hold the Menu Key down until a box appears in the corner of the screen, then press 9,8,7,6 Enter. this takes you to the service menu. Just scroll through the items to find Horiz Size and Vert Size.
Good luck

most likley they are in the tech menue
you would need the service manul to access the proper proceedure the cause may be caps going bad in the vertical circuit

You should have a screen resulation menu and a vertical size (high) control also on the r/c[remote control] menu.
If not you could have a vertivcal (electrical) problem. Let a technician take a peek at it.

If the red isnt responding via the remote you will need to replace the STK392-110 Ics as the service menu adjustment will not correct it 100% due to the IC being faulty and the red and half the green is controlled by one IC and the blue and other half of green is controlled by the second IC so without changing both ICs the set will progressively get worse. Both ICs are stk392-110. You can get replacements at the following link along with instalation instructions
http://servicemanuals.vstore.ca/

Many RCA's use the following to gain access:
Hold the 'Menu' button while turning the set on.
The code may also be inside the back cover.

Yes you can adjust the vertical size in the service menu, but you must be carefull when doing any horizonal adjustment as this can shut down your set .

to get into the service menu using the remote turn the volume down with the button on the front of the tv ( do not release). Whilst holdind tv vol- button press the button on the remote with a circle and three text lines for a few secs. This should let you enter the mode.