Sorry, i can't see any problem. If this is not what you want, how should the panel look in your case?
If your panel should have a specific size, use dip and forget the layout size in the designer (sorry for the bug in my last post, 50dip isn't 9 millimeter, it's 7,9 millimeter).
If your panel should suit to the display, use %x/%y and/or anchors.
Or you can mix, you have all possibilities.