Mountain bike saddle rails extension Please give me the benefit of your expertise.

I'm 6'3" (190 cm) tall and, though I have the largest sized-mountain bike available I'm bent over as I cycle because I can't push back my saddle any more on its rails.

Is there any way that to remedy this situation?
Your best bet would be to get a proper fit at a bike shop, that way they can tell exactly where you are stretched that you shouldn't be. It could be a simple fix or a more complex one.

The confusing part of your question is, if you moved the seat back further wouldn't you then need to be bent over further to reach the bars? Maybe the solution is moving the seat forward on the rails if your too folded up..

Or look into a lay back or offset seat post or stem size/angle adjustments.