I have 30x40, went with 20 foot walls so I could lift my one ton service truck way up on a extra tall bendpack lift. The tall wall also left me with a healthy space for mezzanine, I built an L shaped steel mezzanine that give s me about an extra 600 sq ft of storage area.

In a "working shop", I am not a fan of in floor heat, it means you can't pin big equipment down the to the shop floor in the future. I have had floors in the past x'ray'ed and thermographed trying to locate in floor heating tubes and the locators couldn't pin point the tubes exact location or depth. If you do a lot of welding and grinding, I would forget about floor coatings as well, they get marked up pretty quickly.

I am planning a new shop, this time 30x100 with 4 bay doors instead of 2, with the doors on the long side instead of the short side like last time. I grew out of the 30x40 pretty fast once I took on some larger tools, I should have gone twice as big initially and had more windows put in. I also need more separation between my machine tools and my welding area.