Hi Grandi

I put a font resize feature on all my sites as I believe it is important.
I have seen modules that do this but more likely its a skin object of some kind.

You should be able to see the relevant code in your skin.
If you open your skins "index".aspx in a text editor at the top in the register area you may see any third party skin object such as XFlex.and its path defined here.
If you look in the area of the skin in which the on page controls are located you should be able to see the associated code.
I know that some skins came with Flex in the past.
I stopped using Xflex as it sometimes seemed not to retain the users choices after a site was upgraded.

If your skin is recently made it is likely that designer uses the DNN Widgets object.
This lets the user switch between a set of css files that determine font size and other parameters.
In this case you will see something like TextSizeWidget or more fully (if it is not removed by the forum security)
<object id="TextSizeWidget" codetype="dotnetnuke/client" codebase="StyleSheetWidget" declare="declare">

XFlex and DNN Widgets hold the users type face selection in a cookie. If your browser does not accept cookies normally the choice will not be remembered.

I hope this helps

Ian