Get one built as big as you can, even straining to make it bigger than you think you need now.
My shop was built as a pole barn originally with two thirds of it with a dirt floor and then the former owners added onto it three times, I had the dirt floors converted to concrete and added insulation, electrical, windows and drywall as well as several sky lights to make it useable for me
I would like to add onto it again and double its size, it is right at 30x40 ft. now with the 15 Ft. wide center part with 14 ft. high walls and a 15x14 Ft. roll up modern metal door. the two outer sides have 7 Ft. high ceilings with sky lights in them now.
You know the old saying, a man can never have too big of a shop, correct