if I have a square room of 4m with height 3m, how can I model it in openGL? and what if in the room, I have a cube box of 2m? how can I keep the various aspect proportional? and how can I represent meters in openGL?

Basically opengl is like a big carteasian grid. The units along the grid can be anything you like. You can say 1 unit = 1 m and as long as you are consistent in your units things will turn out preportional