What I've always been told by people who have money to build them, for long term happiness, is you figure out what size building (garage, shed, pole barn, etc.) you THINK you need, and then add at least 50% floor space (ideally, double it). So if you think you need a 2+ car garage, build at least floor space for 3. You can leave the third bay without a door (but framed for one) for a workshop, etc.... if you build exactly what you THINK you need today, you will wish you built it bigger the day after it's done.