With only a floor plan to reference, the designer would know what about a window.?
-the height of the window
-the width of a window
-how far the window is off the floor
-the size of the window

They would only know the width of the window. A floor plan is like a top down view of a building. Imagine that you cut your house in half horizontally, and then looked down at it. All you would see would be the width of the windows, not the height or anything else.