Say what you will --- I have experinced bearing failures twice. These were two different trailers. One a utility -- the other a boat trailer. I'm a PM freak and even tend to over-do some maintanence. I carry two(20) complete spare hubs for my boat trailer and my 5'er. Set includes inner/outer bearing assembly in a pre-packed hub including lug nuts. Call it overkill, but I've been able to get back on the road in as little as 2 hrs without hunting for parts. I also want to address the coment about TSC --- Thier parts ARE probably made in China. Before youe leave the store, remove the bearings from the packing and inspect them. At least that way you know there aren't any "bad" spots in the chinese bearings.
As for checking the size. Simply remove the old bearing and measure the inside with a Micrometer and/or the axle itself