Make it as big as you can possibly afford and what the local law allows. In just about everyone's garage build, the law limited them, not the money.
Try to build in multiples of 4 ft. I am building a 26ft wide, by 44 ft deep. I would have made the building 28ft wide, but the local law restricted me. The extra 2 ft on width would not have cost me much, since I had to buy 28 foot wooden ibeams and cut 2 foot off of them. I also had to cut 2 foot of the sub floor advantec and just scrap it, as it was all tonge and groove. My point is, you end up throwing away alot of short cut material when you do not stick to 4 ft intervals.
Also if you do roof trusses, try to use a steep pitch 6/12 or steeper, and have an attic room truss built. It only cost me an extra $250 to do this, and I got a 30x10 ft , almost 8 foot ceiling room out of the deal.