Each wheel bearing will have the part number stamped on it usually one letter and about five numbers. The inner and outer bearing may be the same or may be different so pay attention when you disassemble. The races and grease Seals will also have a number but if in doubt wipe most of the grease off and put them in a plastic bag and take them with you.
If you have never worked on wheel bearings before please have someone there with you for reassembly because if it's done even slightly wrong you can be catastrophic on the road later.
If you can't make out the tire size on the outside side wall due to curb rubs and or dry rot try looking at the inside sidewall. May have to unbolt the wheel off the axle or roll the trailer forward or backward to do this.