Choosing correct thru the wall A/C-heat?
 

I am finishing out the bonus room above my garage. Space is 530 sq.ft. with 7' ceilings, with full bath and closet, for a guest room. When used, the room will normally only have 2 people. I have put 10" insulation in flat ceilings and 6 1/4" (R-19) in angled roof ceilings with air baffles. R-15 in vertical walls and knee walls backed with 1/2" insulation board. I am planning to install a thru-the-wall a/c heatpump with backup elec. heat. From the a/c sizing charts, it looks like I will need approx. 12,000 to 13,000 BTU for a/c. Will a 12,000 btu be the right size or should I go up the next size to 14,000 btu? I am going with thru-the-wall because I can't run any ducting from the central a/c-heat from the main part of the house. I just don't want to undersize or oversize this unit. I have already run a new 30 circut for this. I live in central Illinois weather and the roof gets full morning sun on one side and afternoon sun on the other side. Thanks for you help or suggestions.

Can anyone give me any guideance on this? Thanks for you help.