[adrianmonk]

If you can afford to pay more, just get a two-stage or variable-speed air conditioner. This is the exact problem they're meant to solve.

They have compressors which can adjust speed, not just turn off/on like a single-stage AC compressor does. So there's a lot less worry about whether it's sized right because it basically "resizes" itself as necessary.

The two-stage type has off/low/high, so it can get a lot closer to the right size. The variable-speed type is essentially continuously adjustable, so it's like always having the perfectly-sized AC unit for whatever the weather happens to be like today.