FS9 Misc page 1 has a set of extra long contrails. it's mainly for AI and default fs9 aircraft, but it probably wouldn't be to hard to adapt them to FSX AI or default flyable aircraft.
"I reject your reality, and substitute my own" Adam Savage, Mythbusters
While they aren't exactly longer but they certainly look better, the freeware REX contrails that used to be available are nice. If you can find them somewhere (oddly not on their site any more..) then they give a nice puffy 3D contrail.