Seasense Bearing Kits

How can you tell what size you need with-out taking the old ones to the automotive store? My Boat Trailer has E-78-14 tires.....

No problems installing the bearings. Had a shop do it. I'd watched several how to videos on how to get the old races out and replace with the new ones from this kit but I didn't have a clean shop in which to do the project (and apparently its critical to the life and health of the bearings that no dirt or grit or any foreign contaminant gets in them). I was working out doors so it would be very easy for them to become contaminated. So I took this kit to Moriches Boat and Motor and they installed them into the hubs for me. Cost a half hour of labor, about $56.