I'm not sure what legal sized items you're talking about that would fit in those size measurement mock ups you see outside of gates. If everything went by those sizes, even 18 inch rollers would be too big.

Some are 21 inch but with obnoxiously big wheels, that might not fit in wheels first I guess. I use a commonly found 21 inch Samsonite spinner that has never not fit into a standard sized overhead bin before.