Experimental only needs to be plainly visible by the occupants. It is written big on the inside.
N numbers are the exact minimum dimensions. I can't think of why that may be.I think it has something to do with insurance.
HiTech
I don't really want my numbers to be very big, either. I think size has to do with speed, also. I remember reading something about how, as you pass certain tiers in speed capability, the minimum size increases.
When I am finished building, I am going to make my experimental decal as large as allowable and put it on the outside, right above the cockpit door..... on both sides

Passenger warning plate will be just as obvious

How long did your build take?
I'm looking at about 2500 hrs total (scratch build)