Bill, you would be best to measure the hub after removing the dust cap. That said, why would you want to put bearing buddies on a trailer hub that doesn't get immersed in water. The idea or concept of bearing buddies is to allow expansion and contraction of the greas inside the hub when the hub is immersed in water like a boat trailer after a long trip to the launch.
Your trailer may have EZ Lube hubs. Have you checked? Both EZ lube and bearing buddies can be the cause of a blown rear seal if not used properly so be careful.