How big is the average trailer wheel?

[showjump2003]

Next week me and my horse move to livery along with the trailer. I have just been to argos to buy a wheel clamp for it and am now thinking I should have maybe checked how big the wheel was. This clamp says it will fit a 15" tyre, that's not that big is it?!

 

[Fifty Bales of Hay]

I think they are 13" generally, or the Ifor Williams are.