When looking at the specifications of the HP Compaq DC7700 SFF Desktop PC, all they state is the motherboard form factor is, 'Custom'.
Same thing for the DC7700 USDT Desktop PC.
Both of these desktop computers are of the 'Pizza Box' design.
(Flat rectangular box)
The DC7700 CMT model is a tower model, and the motherboard form factor for it is uATX.
uATX can also mean Micro-ATX.
The form factor of a Micro-ATX motherboard is 244mm by 244mm, or also said as 9.6 inches by 9.6 inches.
(9.6 inches is approximately 9 and 9/16ths inches)
To regress for a moment, and explain the term Form Factor.
This term is supposed to apply only to motherboards, but has gone on to apply to computer cases, and power supply's.
Form factor means the dimensions of the motherboard, and also where the I/O area is located.
[I/O = Input/Output
Area for the Input/Output devices that are attached to the computer.
Examples: Mouse, Keyboard and Monitor]
Whether the motherboard is installed onto the Left side of the computer case, and the I/O area is on the Left side also (ATX , Micro-ATX, and uATX form factor),
whether the motherboard is installed on the Right side of the computer case, and the I/O area is on the Right side also. (BTX)
ATX is 305mm x 244mm. (12 inches by 9.6 inches)
Micro-ATX is 244mm x 244mm (9.6 inches by 9.6 inches)http://en.wikipedia.org/wiki/Computer_form_factor
Above the link is the Maximum size for the Micro-ATX form factor.
It can also be the Minimum size of 171.45mm x 171.45mm
(Or 6.75 inches by 6.75 inches.
6.75 inches = 6 and 3/4 inches)http://en.wikipedia.org/wiki/MicroATX
Point of all this?
The motherboard form factor inside a DC7700 SFF is probably a uATX form factor, and is 6.75 inches by 6.75 inches.
Or some variation thereof.
It's a custom size.
This means good luck in finding a computer case you can buy off of the shelf, and is large enough to install an ATX power supply.
You can get a custom one made, and you're probably looking at $150 to $200.
Or, you can modify one of these computer cases VERY easily,
The above is an Acyrlic plastic computer case. It's approximately 1/4 inch thick for the case walls, bottom, and top.
It has metal Standoff's that are set up for an ATX motherboard.
It should have holes in the case where a Micro-ATX motherboard can be mounted, by unscrewing the Standoffs, and screwing them into the Micro-ATX holes.
[A Standoff is a piece of metal that is hex shaped, and has a threaded hole in one end, and a threaded end on the other side ]
If there isn't the proper holes for mounting your motherboard, you make the holes.
The case is plastic.
Uses an ATX power supply.
The cost of the case is $50