You've got a loaded question there, and there are multiple answers...
I can't say what every make of bus is in width, but I'd guess they are all about the same. Mine is 90" inside wall to wall...
Length is variable, but you can figure about 25-26" for each row of seating... this is not an exact measure, but general rule of thumb... Stand outside and count the windows and you can guestimate the rows of seating (each row gets a seat). Mine is a 12 window/12 row rear engine Thomas, interior length is about 34' if I remember correctly
The back of the bus has a seat that goes all the way across, and that is set on sheet metal, behind that is the motor. it is NOT flat by any stretch of the imagination.... below is a picture (sorry, best I can find right now) that shows the contour of the rear seat area with the seat removed...
As for the wheel wells, that too will vary on the make. I forgot exactly where they measured out to (it's been nearly 4 years since I laid mine out), but I remember the wheel well being 24" in lenth front to rear...
I placed closets over both the rear wheel wells, and the rest of the rear space I was able to fit a Queen mattress sideways with an inch or so to spare... The floorplan I started with came out extremely close to what I ended up with...
