That's pretty much it. No matter how big it looks once the foundation and slab are poured, it won't be big enough!
Of course, zoning rules have a say in it as well. I wanted a bigger shop behind the house but can only have 1,000 sq ft of external buildings so,,,,,,,,,,24x30 on the shop which left enough for a 12x16 storage shed. Could have gone bigger if it was attached to the house and the attached garage (only 1 car sadly) but my wallet said otherwise. Detached can go on a floating slab, attached would require a 4 ft deep foundation and probably blasting to get through the ginormous rocks.
Anyways, make sure to site the building so you can easily get in and out with a vehicle, plan your electrical in advance. The, insulate with fiberglass and put rigid 1/2" over the studs, followed by sheetrock (for fire resistance). In your climate you can probably go without heat but if budget allows, A/C will make working a whole lot nicer experience on those hot Texas summer days.
Lighting. Figure out how much you think will be enough and then double it (seriously). If you can put windows in, all the better. Mine only has one small window and it sucks (planning on adding some eventually). As I said, budget is the deciding factor and the reason mine was built with only one window. Had it built 3 months after buying the house so money was tight. LOL, like it isn't tight now!
Do what you can yourself to cut costs. I had the excavation, concrete and exterior put up and finished but did the electrical, insulation, sheet rock, etc and paid myself nothing. Oh, obviously I had a professional put the hanging furnace in. One last piece of advise. Finish the garage totally BEFORE starting to use it. I still have bare drywall and and tape joints among other things still not finished because I started on my car project and never got back to the garage project. Maybe this year.......................Yeah right! lol.