mine were only 10ft, but I ran them down the center of the car while the carpet was out. 17 sounds right if youre running from radio to kick panel, along door moldins, then into trunk (15 would probably be too short)
Depends on where you put the amps. I think mine are 17ft with 3ft extensions. I recommend getting as long as you can afford (not a big difference) so you have more room to play without messing with couplings.