What size unit for a home
Your question is just a little like asking "how big an engine does my truck need?". Like several others I would pony up $50 and get a copy of HVAC-CALC, and use it to do a load calculation. That's what I have done for my house and it gives me a certain amount of confidence what is the right size heating or AC unit.
The program is easy to use, as promised. There is a whole thread about HVAC-CALC with generally high praise. There are other programs out there but I don't know of one as easy to use at this price.
One thing I have learned, is inadequate AC performance by no means tells you the unit is "undersized". Most of the time it tells you there is a duct problem or other craftsmanship issue. Makes the problem thornier but different, maybe cheaper but definitely more work to solve.
Once again, I cannot emphasize enough how important it is to get a load calculation done. If you really don't want a do-it-yourself program, then get a few HVAC companies to do a load calc as part of their bid, before you buy anything else. And keep your "baloney detector" working <g>!!!