My inclination is to always go BIG! As big as I can go. And that tendency gets me in trouble because I almost always bite off more than I can chew.
Back in 2001 I wanted a greenhouse and after building a few, first out of PVC and later out of lumber, I decided to go all out and buy a greenhouse kit. We researched and ended up buying a 30'x50' monster. It was huge and a sight to behold. The problem was, because it was so gigantic, it was also difficult to heat. During an extreme freeze event our NG heater light went out and we ended up losing a bunch of our plants and were so disappointed. But we never got even close to filling that greenhouse up and I doubt we ever would have.
So when I recently decided to get back into the greenhouse mode, I decided to start again with something more small and manageable.
Pictures of our greenhouse from 2001:
