I have a trek 3900 that I am looking to replace the fork on with a rigid one. I have been looking online and the suspension corrected forks range anywhere from 410-450mm in length. Would there be any signifigance in handling between the lengths and which one would be best? The fork on now has a length of about 450-455mm when the fork has no pressure upon it.
The conservative answer would be to measure the axle to crown length while you're sitting at the bike, and then buy the rigid fork that's the closest match(or a little shorter) to this value.
My personal take is that if your bike has a pretty average frame geometry, then fork length isn't that critical in terms of rideability - it's highly unlikely to make the bike unrideable whatever you do.
For the discerning rider it might even be worth the effort to deliberately go with another for length in order to tweak the ride characteristics one way or another. Do note that going to a longer fork will increase the load on the head tube-to-down tube junction primarily, which your frame may or may not be up to.
I did the same thing to my Diamondback that I ride exclusively on the street. I saved about 3.5 lbs. and got rid of the annoying pogo-ing when pedaling hard. I used a Kona Project 2 fork from Bikeman.com.