You might want to have a look at my "Dealing with heat issues" thread. (in my signature)
I flower in an outdoor shed, and I live in the desert where outdoor temps exceed 100 degrees daily.

I'd go bigger if you can afford it. Larger units are usually a tad more efficient during the heat of summer, and it's best not to have a smaller A/C on full-blast all the time. (electricity and wear-n-tear)