i think the manitou pro x i have now (what came with the bike) is like 60mm
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Do you have the BLUE FSR?
If so, you can safely go up to 80mm with very little noticeable difference. If you go up to a 100mm travel fork, it'll slacken the head tube angle, and the bike will steer noticibly slower. Not a bad thing if a lot of your riding is fast fireroads, but will suck if you ride tight twisty singletrack.
If you really want to upgrade your bike, you can get a new rear shock and link plate assembly and increase your rear travel to 6". Then, a 130mm fork will work just fine! The company that makes these is called MRP,
mountainspeed I cruised their website and they don't have any info on this for your model, but I know they Used to make these. Send them an e-mail to see if you can get one still.
BTW, I had a '98 FSR Extreme, the neon green one. Same frame, and I wanted this upgrade, so I know they did make one at one time!