I have a 30x40 with 12' eave height and it has an 8' roll up door and a 10' roll up door. I added a 24' section on the back of it to park my boat and camper under (14' for the camper and 10' for the boat). I think the size is pretty good. I wish I had insulated it from the beginning because it is pretty loud when it's raining. Other than that I am very happy with my shop.
Here is a picture of the original construction (I poured the slab myself to save money)
And the 2nd picture is the only one I could find of the addition I added on the back side... Picture was taken during a downpour
I still plan to put another culvert and driveway for the camper and boat but I just havent done it yet...