Most trailers have an axle
and hubs which use what is called a zero-offset rim which centers the wheel over the face of the hub.
Additional to the rim diameter, you will need a width.
Finally, you will also need the correct lug pattern so that the new wheel has the right number of holes at the correct spacing.
It will be something like this: Hub and Bolt Pattern - 5 on 4-1/2" for 1/2" studs.
You could check with Parkliner
, wait for answers from other owners here and hope that theirs match yours, look for your (Dexter?) manual for the axle
, check on the axle
"beam" between the wheels for a label, or take the wheel to a tire shop.
I hope this is helpful.