Trailers have been discussed previously, and very thoroughly, I might add.
My solution to all trailer problems has been to make flatbeds out of any trailer. It doesn't take an expensive frame to do it.
I mounted two 4x8x3/4" plywood on the frame and covered with carpet -- our local carpet store throws out carpet every friday -- That way you can load any size boat, or you can use it for hauling other stuff as well