i just recently put together a 4 player mame showcase cabinet, just the controls you described (although i left enough space to add a trackball later) and mine panel is approximately 45" wide (at the widest point) and 19" deep. it is a non standard shape, but that is the shape of the panel that came with the cabinet, so i just used my router to make a copy of it. it works well and the curved shape is good for players 3 and 4.
i wouldn't want to go any smaller than this, it is plenty of space for 4 adults to stand and play, but players 3 and 4 have to stand at a steep angle.
players 1 and 2 are spaced far apart, more than they would need to be, but this way i have plenty of elbow room, and space for a trackball in the center.

it isn't finished yet, but you get the point. the buttons across the back are the four start buttons, the one in the center is the pause/exit button.
the coin buttons are mounted along the edges of the control panel box to keep the panel clean
at work, we have a similar 4 player panel, but it is 54" wide x 18" deep. it also has a trackball, dedicated 4 way and spinner. the extra width of the panel is nice, and works well with the 36" CRT, however it was just too big for my 27" cabinet.