Size has no real meening when it comes to 3d programing. It basicly comes down to as long as everything is in the correct scale to eachother the units makes no differenct. What give you the concept of size when look at the monitor is your mind telling you how big things are in real life. I.E. put a person standing by a plane, it gives you perspect on how big a plane realy is.
Think of it this way, if you were looking at a scale train set threw a very small minture window, or if you were looking at the exact same real train threw a real window. The picture you saw would be indentical.
The size of your monitor basily determins the window size.
HiTech