15mm D. x 6mm wheel, based on Lego part 50862, so it can be used with 21mm Lego motorcycle tyres. The tyre currently fits a little loose, but I'm working on it to make ita better fit.
Has a hole for a 3mm D-shaped axle, so it can be used with N20 motors and the like.
I personally created the wheel for use in UK antweight combat robotics, but it should be useful for any robotics project.
(Sorry about the terrible photo)