Shoot a popup camper would haul an upside down raft quite well...
3 1/2 to 4' is about the standard tongue length for most trailers of any size.
The tongue needs to be about (preferably a little over) half of the width of the towing vehicle to allow tight turns...
Keep in mind that a storage box on the tongue may interfere with turning.
As for the segway on building a trailer.. I would suggest a longer deck instead of a longer tongue.... Sturdier, more cargo area, more versatile.... Tho heavier and more expensive too....
EDIT: oh oh oh.... How about a built in telescopic tongue.... Able to pull out 2, 4, even 8' as needed and pin in place...
Short for max capacity and long to haul that freak 24' canoe home to the wifey...
Sorry coop I'm easily distracted tho I suppose you COULD apply this to your trailer.