I have been wondering the exact same thing. How do people come up with the size that they did? For myself, I ended up at 34' wide. Seems like an odd measure, but I knew I wanted attic trusses and the truss builder I found could only build 34' clear span attic trusses. So that's what I ended up with. i also wanted to be wide enough to have two vehicles in the garage with all the doors open and still be able to walk around them. I think I'm close.
As for height, go as tall as you can. You don't have to use the space, but you can't easily make it bigger later. My neighbor built a 40x50x14 and can't get his travel trailer or his ocean boat in the garage through the 12' door. I quickly decided to go 16' high, with 12x14 doors. More money, but worth it in the long run. As for length, I wanted the truck and travel trailer to fit inside comfortably, so I ended up at 60'. I am rural on 5 acres, so I don't have any size restrictions.
Good luck with your build and keep giving updates!