As said above ....
You most likely have a 5 lug, 15" wheel - but if it was possibly a special order, it might have heavier axles (5200#) along with a 6 lug, 16" wheel. No real way of telling without seeing the trailer. Additionally and just to be sure, the serial numbers on the axles (or, if lucky a tag) and a call to the axle manufacturer will confirm what capacity you have. Dexter axles have that number stamped on the back side of the tube at about the center.
As far as purchasing a wheel and tire, once you know what you have, any dealer or even an on-line trailer supply can sell a standard wheel and tire for a spare - but be really careful of the brand as some - are called China bombs for a reason
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')