I have built a few. Best thing I have done is gone down to a busy ramp, and found factory trailers for boats similar to mine, and looked at how they were built. You can get a good idea at the dimensions even without measuring, by eyeballing how far the tongue extends (extend that length by a foot or more, as factory trailers are rarely practical. Nobody complains about a long tongue when they get to a shallow ramp). Look at how far the fenders are from the sides of the hull, and what sort of crossbar spacing there is. Realistically though, most bunk trailers are the same general shape, leaving dimensions up to you. One thing you are stuck with is the side bar spacing, depending on what axle you choose. I usually make the side bars a little under even, or about even with the sides of the hull, so then the fenders stick out a bit. I tell ya, those steps forward and back of the fenders are real handy, and if you go tuck them under the hull, they lose that handiness.
If you have any more specific questions, I imagine I could lead you down the right road. Good luck.