We live in an HOA but also on acreage, lots of detached RV garages in our neighborhood. I think you need to push your builder for a 12x14 door. The smaller size might be fine for you, but anybody with a 5th wheel will probably want a 14' door, and 10' is doable, but such a tight squeeze, anybody with a larger rig wouldn't like it that much.
We actually looked at a house that had a nice outbuilding, but the door was too short... made it a big negative for us. What was extra annoying was there was room to put a bigger door, they just didn't because they only had a boat.