I have only allowed 18" behind my machine, from the back bar to the wall, the frame itself with front bar down is 40" deep give or take an inch or two, add nine more inches if you raise the front bar, and two feet of space to work in along the front edge of the frame.
I do have it set up with 10' poles but you can purchase or cut those poles down to whatever size quilt you work with the most for space going the other direction.
