I own a 12 ft Jon Boat with matching trailer. I want to install something like "Bearing Buddies" on the hubs of the trailer.I've looked and there are assorted sizes available,based on the hub size. How do I determine which is the correct size for my application? How easy or difficult are these to install?

Take one of the bearing covers off and measure the outside diameter of it.

As for installing the new Bearing Buddies or what ever brand, You will just tap them in, just don't beat the crap out of them and dent the outside of it all up. I use a brass hammer, because I have one, but you can can tap them in with a regular hammer if you be gentle. You can also place a piece of 2x4 over it and hit it with a larger hammer. Just be sure you don't get it cocked to one side, it has to go in evenly or it will get stuck and if you keep trying to drive it in cocked, you will just beat the crap out of it.

Go ahead and get new seals and pull the wheels off, remove the bearings, clean and inspect them. If there is any pitting in the races or bearings, replace them. Actually, you can buy the bearing kits so cheap at Wal-Mart, Northern Tools, etc, it would probably pay you to get two kits and just replace them.

Since it seems you have never done this, it might help to get with someone that knows how, so they can show you how to get the races out and install the new ones. when you install the nut, DO NOT tighten it down. Snugg it up and then back it off approx 1/8 turn. You should be able to feel a very small wiggle/movement in the tire when you move it side to side. This is so there enough space between the bearing and race for a grease film, it will overheat and burn out the bearings if theres not a small clearance for the grease film.

I use a short piece of 2" PVC placed over the bearing cup to tap them on. Bearing buddies might require a slightly different size. This will prevent you from denting them while beating them on.