The only other idea I have is take the measurements from the rim but I don't know how to do this. If someone can tell me how I can figure this out that way.
If you're going to replace the wheels with the tires, you need the bolt circle for the lugs, which you can measure. Most common for trailers this size is "5 on 4-1/2", or 5 lugs on a 4-1/2" bolt circle. You probably don't need it, but you can also measure the hub hole diameter.
To measure the width of the wheel, for replacing just the tire (wheel width is usually given in tire specs), lay it flat on the floor, and put a straight edge across it. Measure tire thickness to the floor. Then measure from the tire surface (bottom of straight edge) to the wheel flange where the tire bead sits. Subtract that twice for the wheel width. The diameter is already known. You only need this if you're putting new tires on old wheels, and you're not sure what kind of wheels you have, or even what kind of axle it is (could be an old car axle for all you know).
Many places sell trailer tires and wheels. I got new tires and wheels (already mounted) from
www.easternmarine.com, but many places sell them, including local suppliers. I recently found a small trailer dealer local to me with competetive prices, so I'll be using them for such things in the future.
If the wheels are in good shape, just put new tires on them (with new valve stems), but if they're rusty, bent, or otherwise in crummy shape, new tires on galvanized wheels are usually less than $100 each when you hit the right sale. Eastern Marine has ST205/75D14's for $61 each right now, and $75 each for the same tire mounted on new painted wheels. Watch the shipping, as some places charge more with 'free' shipping, while others charge separately. Or buy local.
From your pic, the wheel looks like the old-school automotive type wheel, common back in the day. It will even take a hub cap. They used to make them with a hole in the center for Bearing Buddy access.
