Suggestions I'll add:
Go as wide as you can without interior support posts. Mine is 24' wide, no posts, and often it is REALLY nice to be able to park my car diagonally across the garage, plenty of room, especially when I do the occasional detailing/polishing.
LOTS of lights.
Ask around to find a good concrete contractor for doing the slab. You will want it very flat (no low spots), and with a slight pitch so water drains out the doors. My garage slab has a low spot right at the front left tire where I park my car, it always puddles water there when I bring the car in from the rain or snowy weather. I hate it.
Make sure the slab mix DOES NOT have any calcium chloride in it. CaCl is used as an accelerant many times, but the problem is it doesn't fully react with the concrete mix, it's only there to add heat by hydration. As the moisture cures out of the concrete, the CaCl will re-crystalize. Then when you bring in your wet car, it will re-hydrate, and accelerate corrosion of your car.
Think about if you may want to have a lift some day. Make sure the slab is thick enough to support it.