I just ordered the same bike. When I test rode it, the salesman told me the same thing. I went with the large frame i'm 5-9.5.
The seat felt low all the way down to me, so I fell like Ill be fine. I think youll need to do a fit to see how it works out.
Found a quick guide, im sure a quick search will turn up more.
Enjoy it!! I cant wait!
Technique: Perfect mountain bike fit - BikeRadar
Keep in mind if your doing difficult trail, you may want to drop the post lower for descents, this is where it might be nice to shorten it.