Proper fork length for a bike with 5.5 inches of rear shock I purchased an older Santa Cruz Blur frame today.  From what I can gather the shock mounted on the frame gives 5.5" of travel.  I'm looking to buy a fork to match.  Do I need to go with a 140mm fork, or would something like 120 work?  What are the problems associated with differing lengths of suspension between the front and rear?

Thanks for any suggestions.
The different lengths of suspension will affect the geometry by giving your bike a steeper (or not) head angle. Too much travel might make it tougher to pedal as well, depending on the fork's settings. A too long/short fork (compared to what it was designed to use) might put some unnecessary stress on the tubes and for that reason forks outside of a certain range void warranties. 

The blur is designed for a 5.5 in / 140 mm fork. I'd suggest going to a local Santa Cruz dealer (if such a thing exists where you live) and ask them if 120 mm is fine too.