Unless your cloest is 8-9 feet high, you can't grow a plant 5-6 feet in there. You have to take into account the pot height (with a plant that size you would need a 5 gallon bucket, maybe more) and the distance to keep the light away as to not burn your plant. Plus, as the plant grows taller, it usually grows wider to. So the closet would also have to be around 4-5 feet deep in order to house that one plant. BUT if you could do it, I'd expect 2-3 lbs off a plant that size.
That question is almost impossible to answer. There are so many variables that can impact harvest. Growing ability/knowledge, nutes/additives, growing tecnique (SOG, LST, ScrOG, etc) genetics, temp/RH, method of growing (soil,aero, hydro, DWC,etc).