Thanks, I had a look too and there is a wire on the back of the motor that could prevent the largest tyre sizes, however looking at mountain bikes outside I realized there should be enough room for cross country trail bike tyres (XC) - I doubt fatter tyres would be worse (for efficiency and speed) than badly aligned disc brakes on the current tyre size.Plenty of room in the rear for much wider tyre, 650b rims ,think the front is 1,1/8 fork unit again easy to upgrade and plenty of room for wider rim in std forks which are a little on the soft side.
Heavy duty forks are fairly important to my style of riding, although the suntour NEX forks are much nicer to ride on than the fixed aluminium forks I have on my road bike. The roads in my area could probably cause concussion for a roadie.
I think it might be interesting to account for the possible change in speed for smaller wheels too, that would allow more options for tyres and forks. The hall magnet could be aligned to the same distance on the smaller wheels.