I'm just using the Default Xenforo Style and made a single child of it, which I called +SZ.(for Plus Size)
In looking around, I thought it might be here (Appearance > Styles >+ SZ >Style Property Groups) so I tried 20...
and then 20pt, but I still can't see any size difference in the text for this new style.
What am I missing, or where do I need to be looking?