Mine will only be 28x40 with no room to ever add on...
The biggest thing I noticed I needed was a place to put parts that were cleaned, painted and ready to be installed that was separate from where I was usually working and making a mess.
I set mine up to have a an 18'x28' working bay and a 22'x28' bay with a four post lift for storage of finished cars and parts. I am putting a wall between the two with 2 pocket doors that will leave me a 4' wide opening when opened and no doors to swing out and hit a car. Underneath is an 11x38 basement where the in floor radiant heating system and air compressor plus all extra storage is located.
It's not much and sure not very big but I'll make it work...
I currently have 4 cars and if I was to get another one has to go...