most measurments are rough measurments
like a tub is 60" or 5'
but after its rocked and tiled
1/2" rock
1/4" tile
1/4" grout= 1" on 2 sides= 58"
tubs generally are given 32" when figuring out layout
a toilet space is 30" minnimun to 36"
cabinets are measured as they are 24",36",48"
so..a shower is 36 x 36 thats rough or frame demision
but...when actually framing for a tub or shower
make the framing 1/4" bigger than it actually is
because.....a 36x36 shower will be tight as he$$ if you try to put a 36x36 in a 36" hole..its just common sence stuff. have fun

Thanks for those measurments. So the sizes people report are a couple inches bigger than the actual inner space. They are generally stating outer dimensions.

Judging by the standard sized shower pans I'm finding, my current shower is probably considered 32" x 48". Must be why there is an 8" wall at one end doing nothing.

I can easily make it wider than 32" (by taking down the back wall), then add the 8" available space on the end to have a 54-56" length. But going to 60" would mean taking down another wall I don't want to if I can help it.

Okay, have to think about this.