Just depends what you need to roll over. Never did them to roll around a car, but plenty of carts. A small wheel 4" or so, would be fine if just on flat, especially if smooth concrete. But to get in/out over a transition or any stones, I would want 8" dia or bigger like a wheelbarrow tire.
yeah, my current home built cart uses 5 inch, 1550 lb rated each casters. they are smooth casters and hardened rubber. I'd like a more off road approach so i can wheel the thing outside at times.