Hi, I want to know how big of a ac unit I need for 632 sqft. Guest house.

Square footage is only one of many considerations when sizing HVAC equipment. A proper load survey of the structure will determine the requirements.

I'll assume you want a window air conditioner, right? Every window air conditioner I have ever seen comes in a box with the sizing chart right on it. It's a good start unless you have special circumstances. If you oversize the unit, you will get the cooling, but not cool long enough to get rid of excess humidity... too small and it may not cool enough. Go the Grainger website and look for an air conditioner sizing chart.