hang on, just had a thought, you got 255 tyres on your carlton (sort of). The 255 number is allegedly the width of the tyre is mm. Soooooo
255 / 25.4 = 10 inch rims

Mind you had a measure today, and theres an 11 inch gap at its thinnest point between inner and outer arch. Not that simple I know as the wheel doesnt move directly up and down, more of an arc.
Hmmm Fairly chunky rims should be possible anyways, just need to calculate the offset.