Hi Speedy,
I'm in a similar position - setting up to make a 2 player MAME cab with 19" monitor, and trying to get a feel for how big it needs to be.
I've been reading the 'Project Arcade' book, and the plans in there, though for 2 player, are for a BIG cab (44" deep if you include the CP overhang).
OTOH, looking on jakobud.com shows that many classic cabs were a lot smaller - e.g. 30" deep for Defender, and I think I'm going to go for something closer to that size.
Of course, a general MAME cab control panel is going to need to be bigger/deeper than E.g. a Defender panel, just to fit everything in, but 14" cab depth is a big difference!
I'm looking with interest at the plans at basementboomerang.co.uk as a possible starting point.
As you say you are using a PC monitor anyway, remember that if you use an LCD you can reduce the cab depth quite a bit if needs be.
(I'm still torn between CRT and LCD for the monitor).
Anyway, good luck!
Cheers - Steve