If it's the same welded rectangular tube axle as mine (original equipment), the inner bearing is an L68111 1-3/8" ID bearing, and the outer is an L44610 1-1/16" ID bearing. But if the bearing is shot, so are the races, and to get to the inner, you have to wreck the seal, so the best thing to do is just get the whole kit. And make sure it's a double-lipped seal, to keep grease in, and water out. Like this one. https://www.etrailer.com/Trailer-Bearin ... 2-100.html
I've also been able to find similar kits at the local auto parts store, but I don't remember now if they had double lipped seals.
Here's a picture of the axle, for reference. I had stripped out the brakes, which were stuck and burned as I brought it home the first time. I've since installed disc brakes, and stripped out some of the springs leaves to soften the ride (mine has 2 axles, per a previous owner). But your OEM axle should look like this, rectangular, with the spindle welded above the centerline. 3500 lb axles most commonly, but not exclusively, use the bearings I linked to. Some of them, however, use two bearings of the same size, so make sure you get the right type. Best to pull it apart and measure the spindle OD with calipers if you're not sure.
The shot below is of the other axle, but it uses the same bearings and seals, and is the clearest shot I have of the spindle. The seal rides on the largest OD, the inner (larger) bearing bears up against the shoulder, and the outer (smaller) bearing bears against the shoulder just beyond the land after the threads, where the protective brown filth ring is.