Lots of good advice here. For a fixed garage application get the biggest tank and best motor/head you can afford. Ive been running the same Snap-On 30 gallon horizontal tank with Dayton 110/220 motor and Dayton V-Twin oil-bath head unit for 20 years myself. I bought it used as well. Mounted and plumbed in both of my garages since I've owned it.
Outside price, going bigger would be nice, but being in the military I move around a bit so it being somewhat portable is good.
I think 15g is a pretty good size for me. But this could just be me trying to justify the DeWalt compressor. Just fond memorys of my dad using DeWalt tools.