John,
You'll know when you have everything spot on, the dog will be able to lift it's back leg to have a piddle.
Maybe the larger wheels are required for stability, all you would need to do is make the spindles adjustable up and down, so the trolley can grow as the dog grows.
Have you thought of clip or bolt on plastic wheel discs, like they have on full sized wheelchairs, then you can just paint and decorate them at will. Or do you want knives coming out of the hub centre, like on chariots?
Bogs