I've rewired a car hauler trailer before and my suggestion would be to try and get a rough measurement off the trailer itself. Run a tape down along the length of the trailer, then across the back (left to right). Your wires will come down one side (typically the left/drivers side) and then cross over for the farthest away light (right side).
Roughly figured, if you've got a 20' boat, then your trailer is at least that, will typically be just short of 8' wide, and will have an extra couple of feet in length for the tongue. To start off, you're easily looking at 30+ feet (closer to 35) of wire. I would also get more than the minimum, so now you might as well figure 40 feet. Better to have more than not enough. And the wire will be cheap enough to afford the extra length.
Just my $.02...
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1999 180 Horizon OB Fisherman Pkg
San Antonio, TX
Choice of tow vehicle:
'93 Explorer 4x4 (sold)
'05 Grand Cherokee 4x4 w/ 5.7 Hemi (member of the 240k+ mile club!)