Don't go higher than nema 34's it's not needed.
Like neil says you need lots of voltage with 34's to get good performance so buy mains voltage drives. These are what I use with nema 34's http://www.zappautomation.co.uk/elec...n/2m2280n.html

Seem expensive but you don't need a PSU so it cuts down on price and complexity and they are the only thing I'd fit to large 34's.

Size wise then you won't need massive motors these should work but I would get some idea of weight your lifting or moving first. I've seen nema 23's fitted to X & Y on these before so these will easily do it with some to spare. Thou I think you'll need the extra for the Knee.
http://www.zappautomation.co.uk/elec...118-6004b.html