Ability is the major X factor here. Have you raced before?
I am 5'9" 175. I race on some Jr gal's recycled Fischer 175cm GS skis 23m. They work well in the beer league course (pitch-flat-pitch 30seconds) at Wachusett. Next year I am going with 180s.
Depending on set and conditions, 172 would work but would be a bit short compared to many of the racers on my teams. If you are a dude at 5'8" 150# you are either A: Real skinny or B: Really fit and strong. If you are a woman, then you are C: tall and strong (My kind of gal!)
If B or C, you should be able to easily bend the 172s. If the course is real GS style, longer tends to run a bit faster esp on the flats. If tight beer league style, shorter runs tighter with more margin for error. You MAY even want to consider a SL ski. I know a lot of Nashoba guys/gals run a SL ski. (But that is not GS)
Otherwise, for a new (?) racer - look for a 21-23m radius. 27 is not very fun. Might be hard to find a GS "race" ski shorter than 175. That said, DO NOT RULE OUT A JR SKI!! Guy I coach with swears by them and he is bigger than I am.
Finally - for GS - get a suit. On a 30 second course, it makes a 1-1.5 second diff. This is a LOT.
Where are you racing?