although i have posted this before, i cant find the info, so just to clarify this once and for all, i just got off the phone to vicroads, according to the australian standards which are enforced by all road authorities in australia (i.e. queensland transport, vicroads, the rta) wheels cannot be more than 25mm wider than the largest wheel fitted standard to that vehicle from the factory, the overall diameter of the rim with tyre fitted cannot exceed the overall diameter of the stock rims with tyres fitted by more than 15mm. this means that, the size is dependant on the diameter of the tyre, not the rim.
so, for u12 drivers, you can fit any size rim provided the overall diameter of the tyre is no more than 15mm larger than the diameter of your stock tyres, which has nothing to do with the diameter of the rim, and the wheels are allowed to be up to 7" as the widest rim fitted to the u12 from the factory were the 6" trx wheels.