I've had a 10,000 btu a/c in my garage for 25 years and it works great. I just bought a wall unit from Sears. It's been a long time, but I seem to recall buying an optional hanger from Sears to use when wall-mounting versus window mounting. I just followed the instructions, cut the hole to size, and mounted it. I put a good bead of silicone caulk around the outside where it protrudes through vinyl siding and never had to touch it since. I have a very well insulated (by me) 2-1/2 care garage with a drywall ceiling and paneled walls. The 10,000 btu is just big enough. I wouldn't go any smaller and a little bigger would be okay. I mounted it high up on the wall to discourage thiefs from knocking it out as an entry point and also to distribute the cool air which is naturally dropping as soon as it comes in. I only run the a/c when I'm doing something out there in the summertime. The garage will always be a little cooler than the outside temp if you keep the door closed. I live in Michigan and we get our fair share of 90 degree days here too with brutal humidity. I'm sure we're a little cooler than Texas on average, but I don't think you'll have any problems.

Verne