Getting ready to build a shop- first time in many decades I have done so- perusing this forum for ideas.

The only things I can offer in the way of advice are:
1. They're never big enough. Go as large as you can.
2. If you build them too big, you start acquiring more crap that you don't really need. Or it becomes storage for too many non-automotive things. And people start pestering you to store THEIR crap.
Catch-22.

Edit: you really should have some kind of walls, even if it's just lattice and screen to give you adequate circulation. You want to minimize the opportunity for critters and birds to foul up your workspace. Last thing you need is a gecko taking up residence inside your partially assembled long block!