What size kayak trailer do you need for a 14ft kayak? We are wanting to modify a trailer to haul our kayak and not sure what length we need.
The length depends on how you build it
Kayaks are light so weight distribution isn't much of an issue. The shortest kayak trailers have the center of the boat just about over the axle. With that setup you need the forward part of the trailer (including the tongue) to be long enough that the hitch extends past the boat. Make that setup two boats wide, and you need more length so you can tightly "cock" the trailer when backing up without one of the boats hitting the back of the car (that is, the tongue needs to be longer so it's the only part that gets close to the car when the trailer is cocked 90 degrees), but that's probably a good rule with one boat too, just so you get a few more inches of angle, and no chance of damaging the boat or car.
Other trailers have the kayaks a lot more forward of the axle, and the only difference in that case is that the front end (including the tongue again) must be longer to meet the same criteria as before.
On still other trailers, the rear part of the trailer is longer, so very little or none of the boat's length is hanging "out into space". In that case, the need for front length is the same as before (enough to allow sharp turns when backing), but the extra length that's present to the rear means the whole trailer ends up being longer.
So ultimately, it's up to you.
I haul those on a 4x 8 utility trailer
It’s three tiered and the two upper tiers are bove the level of my pickup bed so I can put the boats as far forward as the ball and even a little further sometimes without worrying about turning corners. Leaves about hree feet hanging out the back with red flags tied to the grab handle.
14 footers not a problem
I use a 5’X 8’ flatbead with bunks. The overall length of the trailer is 12’-6"; with the bunk setup I have, three boats can be hauled on edge. The longest boat must be hauled in the center. My longest boat is 19’-2", so about 7 feet hangs out behind with a red flag.
I would highly recommend finding a trailer with larger wheels and tires and not one with the tiny wheels that have to spin a gillion rpms to keep up with the towing vehicle.