There are lots of options out there... I've done the snowmobile trailer conversion. You can add a roller and extend the tongue pretty easy on most. Just pay attention to how the tongue is designed. Most are simply box tubing running back to the axel and then thru bolted to a bracket. You simply pull the wiring, pull the bolt and buy a longer piece of box tubing, drill a few holes for the coupler, pivot pin and lock pin and finally splice in some more wire. When I did it I went with a heavier wall so it would remain stiffer over the longer span.
I have also converted both boat trailers and utility trailers by adding a raised deck. There are threads discussing both conversions; I started one for my utility trailer rebuild but I can't recall the name and I know nany of us have posted discussions to the trailer porn thread too. Search and ye shall find more ideas...
What I can say is that in my experience having a deck that is bigger than the boat (wider that is) is super nice because you can walk around on the side of the trailer and access the boat. If the deck is too narrow your forced to crawl up over the side or cram toes in for a foot hold when you're digging around at the ramp.
Like Osseous said, length doesn't matter that much, my 15' boat sits on a 10' deck, and what hangs over wouldn't be touching the deck anyways due to the rocker... I just have about 2' off the front and 3' off the back. Don't let your rubber ride on the roller if you have one - it will wear the rubber - no bueno.
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae