If it's any help, I have one of these:

http://www.toyotomiusa.com/products/portableairconditioners/TAD-T32JW.mv

It's 12K btu, and adequate to cool a 10' x 15' room when it's 100°f outside. That's the room that gets the afternoon sun, and I've mentioned the almost total lack of insulation. It's also the room I work in, so we tend to just cool the one.

From my experience, even given that it doesn't get as hot there as here, the 24K btu would probably be minimum for you. Definitely don't go less. I think I'd go for the 36K unit and not have any regrets later.

I threw that gas thing out there more for an interesting read. I worked in an office once that had one, it was way cheaper to run than an electric air con, at that time.