How do I know what size AC unit I need for my home?

Call an air conditioning contractor and ask them to give you a free estimate on installing A/C, ask them to do a load calculation to determine the proper size equipment for your home. Mike

A Manual J calculation determines unit size.
A Manual D calculation determines duct design.

I wouldn't expect these for free. It might make sense just to pay independently to get that done. Then get at least 3 estimates.