bicycle seat height vs length from stem to seat post?

[truth]

My body is out of proportion. This was gleefully told to me by a guy in a bike shop. He's right of course but I conveniently forget it when possible. Anyway on my older Specialized mountain/road bike, when the seat is at the right height I feel I am going to go over the bars just sitting there and did a few years ago breaking two ribs. So I want to lift the stem I suppose but I am told I can not on my older specialized. What should the distance be between the stem and the seat post or is that fixed? Thanks