I like 70' x 30'.... Wall off a 10' X 30" section for the machine shop at one end, mechanical service gets another 10' section walled off, and body work gets another 10'... Seal these sections off so dirt doesn't migrate but put in a man door so you can pass between them.... I actually picked up a few small roll up doors (7' tall & 4' wide) for/from a Mini-Storage to allow free travel when no dirty work is going on... Also planned to use one to the outside of the body work section to allow air movement through the dirtiest section without having to open it to the other sections...
Nice thing about all these walled off sections besides controlling dirt? You have walls to park tool boxes, work benches, shop equipment, etc...
Need a full sized roll up door for the mechanical & body sections, not needed but nice for the machine shop section...
Anyway this still leaves you 30' X 40' for finished cars, equipment. etc....
Light panels in the roof tend to deteriorate over the years, windows at normal height are a security concern... Windows mounted high in the wall provide light & can be setup to offer ventilation...
Put plenty of piping in the ground, water, sewer, electrical, ethernet...
Plenty more to think about...