You might try going (with the pictures) to a boat shop, and see what they have that is similar. You may not get an number that is exactly correct for your trailer, but it should be close.
I have a 4x8 utility trailer, and it's only got a single leaf, and I can get a lot of stuff in it.....so unless you plan on loading 50 sheets of plywood or several hundred concrete blocks, you should be fine with a smaller utility trailer on that axle.
You probably should also calculate the weight that the tires can handle...which may be less than the axle is rated for. That would be your max. weight limit anyway.
Still, you may get a better answer from someone who has a similar boat trailer...sorry, I don't.
