The standard sprocket sizes for the 2v 900 Monster are 15/39 (the 900SS is 15/37 due to different primary ratios).
On my '97 900 I went up two teeth on the rear which makes sixth gear actually usable below 90mph and more importantly prevents the 'in between gears' feeling when riding around town and in slow traffic.
If you want to play around with gear ratios here's quite a good tool although it only lists the 900 from 1999- but it's the same:
http://www.gearingcommander.com/