I like a mottled grey for portraiture, too. And, I often use a "Thunder Grey" (close to 18% grey) seamless paper for other work. The grey, as noted, can be lighted (or, not) to virtually any tone by controlling the subject:background lighting ratio, and takes color from gels nicely, as shown below.
The best size depends on whether you want to do full-length shots, or just head and shoulders. Go for 9x20 for the former, smaller (shorter) for the latter.
For background lighting in tight spaces, try mounting a Matthews Grip "baby plate" on a piece of plywood, so the light can be placed on the floor behind the subject. Then, add a modified reflector, and maybe some diffusion, to better control the spread.
The Matthews Grip Company site