So my roller door is 3.2mhigh (clearance) - and has a box above that for the rolled door (about 600mm).
It JUST fitted my camper in.
So if you worked on clearance of around 3.4-3.5 for most campers/boats you would be good.
That would take you to around 4m or slightly more - depending on door fitment etc for height.
Originally they fitted a tilt door to my garage. No good - lost about 150mm with the tilted door.
If you go to 4m height, you also have enough height for a hoist. Mine comes in at around 3.8m.
You obviously don't have to fit a hoist - but they are not expensive these days. For someone keen on one, that would be a big plus. Most sheds are not designed with that in mind and are too hard to retro fit. So building in the capability to fit one would be a big plus for resale. And at that height you can accommodate most boats and campers.
Alan