For length...
Allow 2-3' for backdrop
allow 5-6' between subject and backdrop, space to set b/g lights and to blur the b/g and not have shadows land on b/g
allow 2' for subject
allow 8-10' camera to subject typical distance, based upon most prevalent FL in use for studio portraiture
allow 2-4' camera and photographer to walk behind camera
Total: 19' - 25'
Height: 8' is restrictive better 10' or 12' ceiling
A 32mm lens captures an area 7' x 4.6' at subject distance of 10', perfect for full length standing or 3/4 length. At 15' (where the b/g is) the lens sees an area of 7' x 10.5', so you understand the reason for ceiling height to support a b/g sufficiently tall to not have its edges captured by the lens.
A 65mm lens captures an area of 3.4' x 2.2', which works for head and shoulders shot at 10', and at 15' at the background the lens will see 5.1' x 3.4'
So you can see how the style of portrait shot greatly influences the amount of space and size of backdrop needed. You can reduce the size of the backdrop by using a longer FL, but that brings a correspondingly long shooting space!