This part is not quite clear:
I allow my users to use a different font size when needed
If you allow your users to set the font size of the actual text, it will override the layout setting. And not just in Preview mode, but in all modes and in all layouts.
Thank you for your fast reply. So, in other words, there is no way to tell a text field "In Browse mode use the user's font size settings, but in Preview mode use the layout's font size setting."
So, I'll use a lighter gray text color instead of a smaller font.
If users are selecting the text in a field and applying a size to it (you haven't really clarified if that's case), then no. These are not "user's font size settings" - they are text attributes applied at the data level.
Formatting in-field text changes the (default) plain text in the field into styled text. And styled text will always override the layout settings in any parameter it has settings for (font size, color, bold, italic, etc.).
You can take away this option away from your users - but once it's there, it'll stick untill removed at the data level again (e.g. a calculation using the TextFormatRemove() function).
If users are selecting the text in a field and applying a size to it (you haven't really clarified if that's case), then no.
...that's what I meant with "user's font size settings". Sorry for having been imprecise.
Merci beaucoup for this in-depth explanation - very helpful!