He means how to make rear-wheels in the model itself bigger. There is actually two ways:
Leave the model like it is now, and inside vehicles.ide, change fourth number from the end (-1, 0.7, 0.7, 0) to 0, and then modify those two other values. Now they will also increase the wheel model size.
Make the front and rear wheels work like a different pair, I don't know how it works, but when you get both wheel-models in-game to be good size, again, play with values in vehicles.ide.
And the model looks great, but is interior looking as good as outside?