Posture on mountain bike long term
Further to the above comments:
Your bike size sounds about right - see here:
Evans size guide
Stand over height is that you can stand over the top tube with your feet flat on the floor and raise the front wheel off the ground. When seated on the saddle you should be able to touch the ground with toes and possibly ball of your foot on one side at least. Look also at having your leg only slightly bent when your pedal is at the bottom of its stroke.

Before ordering a new bike visit a few cycle shops and try different frame sizes and wheel sizes. Mountain bike wheel sizes go from 26" to 29". Some riders prefer a frame on the larger size and some on the smaller size. "Feel" although it cannot be measured is a good indicator of a bike that is right.

Having a bike that is too big may not cause any noticeably negative effects, but handling and efficiency may never be as good as you would want.    

k