As far as sizing it, use the online calculators. Do your best, then double it. I wish I had done that for my house. It's a 1947 vintage with absolutely no insulation (or moisture barrier), and there's days in the summer that the house will heat up to 85°f inside with the central air going full blast. I can only guess that the online calculators were assuming a far better insulated house than I have.

The cool thing about the split systems is that rooms will drop off the air con system as they reach temp.