Steve,
I have one of the Walthers modern roundhouses under construction. Actually, it is the basic roundhouse with the extended stall for my Big Boy, plus the add-on 3 stalls (6 total) and the machine shop. By the way, Walthers shows the machine show attached to the roundhouse, but I don't see how a rectangular building can attached to the angled roundhouse and still have the track meet at the center of the turntable (unless you curve the track somehow). The basic dimensions are: each stall is 17 3/4" long, 5 1/8" wide at the back and about 2 1/8 wide at the front. The add-on section makes that stall 20 1/8" long. The box gives the overall dimensions for three stalls as 16" wide at the back and about 6 1/4 at the front. It also shows the distance from the stall doors to the center of the turntable as 11 15/16".
I have left this part of my layout to last, but am rapidly approaching the point where it is unavoidable. The instructions make a big point about how sensative the turntable is to dust so maybe waiting to install it was not a bad thing. I plan to install the turntable soon and then the roundhouse and machine shop. Hope that helps.
Keep flying with the eagles!
Mentor63