I only have he 85 1.8, so its kind of hard to tell with that
I just did an experiment for you. With my 20D and my 24-70 lens at ten feet from a wall, roughly 35mm allowed me to cover roughly six feet top to bottom in "portrait" position (the long dimension of the frame vertical). With my wife standing in to help, 28mm at ten feet gave me extra room above and below her. I did not like the perspective at ten feet, though.
Backing up to about fifteen feet and using 35mm was a lot better. Further away (and with a proportionally longer lens for framing) would probably be even better yet.
I would suggest that you look at a zoom lens with a range something like the 24-70 or even the EF-S 15-85 to give you the versatility that you would need for a variety of portrait work. Your 85mm lens is a bit long (on an APS-C format camera) for studio portrait work. I would say that you do not need a lens any faster than f/2.8 for portrait work. The zoom lens would enable you to do a lot of work that you cannot do with the 85.