The line of sight thing is just a starting point and in my experience applies more to road bikes than mtb. Of course....if you're looking to ride in the same position as your road bike, then all bets are off.MTDirtGirl said:
Typically, an MTB will be set up so that you ride in a more upright position than a road bike. This will help handling on the downhills, but will be at a sacrifice...to climbing. I usually set up bikes to favor my weaker points....in my case, decending.
It's all personal choice. What do you want out of your MTB? At 5'8" with a 33" inseam (is that 33" pants inseam or "true" inseam?) your upper body is short in comparison. Most people of your size would not go for a large FSR...even a woman's version. If you feel too cramped on the medium, have the LBS put a longer stem on the bike...and / or move the seat back a touch.