Any Matt area designed around a template opening will scale in proportion to the overall template size.

If you wish to have a fixed size space around an opening, then consider instead using a style, because these do not change width for different size templates. In this particular case,

1. Make sure the opening is the full size of your template and
2. Apply a style with a keyline set to your matt color and width to represent the matt area. (Keylines can be up to 5" wide (127mm), and if needing greater width the add two keylines together).

This will work well for viewing purposes and if you are producing the layout as a Composite print. If producing as a traditional matted layout, then it won't print at the size with the keyline space unless you include the keyline as part of the print (Preferences --> Hi-Res Production --> Style options --> Traditional Matted prints--> Styles:Add Borders).

Alternatively you can use the Alt/Option drag and drop template swapping shortcut around fixed size designed templates (a template attribute set in the Template editor) which will replace the layout and retain the same images as the original layout. This particular shortcut information can be found on the "Thumbnail View" page of the ProSelect Reference Manual (quick access under the Help menu in ProSelect).

There is also an article on "Template sizes and scaling", and the pages on "About Templates", "Styles - Editing & Options" & "Styles - Hints for Using" of the ProSelect Reference Manual which may be off interest.