Specify the number of pixels high the pane area of all the dialogs in this "frame" of dialogs should be. You can also click and drag the top/bottom of the pane to the exact size you want, and this property will update accordingly.
TIP: | You can use this property, along with the Width property, to make the pane area of all of the dialogs in your project the same size. Click and drag the outside edges of the first dialog's pane area to the size they all should be, then copy its Width and Height property values to the other groups of "frames." |