Check the hub/drums and see how many holes/lugs are present. These era trailers often have the Dexstar-four bolt open center rims. They are sold at Etrailer or a recycler may have some.
If you plan to keep this trailer for years, you may wish to call Dexter, get the information on the axles, and install compatible 5 bolt rims/hubs. I've been told they are available.
Good chance the wheels were 15".