You would need an unstored calculation field for it to update with a layout change.
But here's a simpler way:
Set up this auto-enter calculation on this field and any other where data may be pasted into it from another source:
TextFormatRemove ( Self )
Clear the "do not replace existing value..." check box
Then, on each layout, select the field while in layout mode and use either the formatting tool bar or the inspector's appearance tab to specify the desired font size and any other format options. These settints will only apply to unformatted text in the field, but are specific to the layout so you can get different formatting on different layouts.
Works brilliant - simple when you know how!