Rough rule of thumb is that the diagonal measurement of the screen should be about the same as the distance from your eyeballs to the screen.
For Widescreens, use the diagonal measurement of the 4:3 image displayed by MAME in the center as a reference instead.
I have a 27" widescreen monitor in my cab and the 4:3 image is around 22". I wouldn't want it any bigger, but my CP isn't very deep and you're pretty close.
It's a personal preference thing. I can't imagine some of these cabs with 32" screens 20" from your face being playable, but some people like them.
If I have to move my head more than an inch or two to keep track of the action, then the screen is too damn big.
On the flip side if the screen isn't going to rotate, you want it big enough that the vertical games don't seem undersized.
Lots of people have used tvs that big and even projectors. It's all dependent on how far away you are.