When you are insetting a face, you can adjust the thickness by moving your mouse. It's very much like using the mouse to scale. If you move toward the origin of the selected face, the copied edges get smaller. If you move away, they get larger.
In lieu of using the mouse, you can also type in the desired thickness. For these window panes, it makes sense to type in a number so I can make sure each and every pane is identically sized. I can use trial and error to find the best size. Another option is to inset one pane and use the mouse to size it. As I work, Blender displays the current thickness at the bottom of the screen. This can help me get a feel for what thickness I'd like to use for all the panes. ...
