The font resize plugin is intended to be for your users so that they can set the size they need should they need it bigger or smaller than your default. It places a font resize control above and to the right of the forum as you can see on this very page. An individual users preferences are saved in a cookie just for them.

If – on the other hand – you wish to change the default size of your fonts – and by default it inherits the settings from your WordPress theme – then you need to make a small change to the Simple:Press theme you are using. If you let me know which theme then I can point you in the right direction.

the theme is NewsPlus

Sorry – I really did mean the Simple:Press theme you are using. There are some differences between them.

Im using the default one

well, its really the overlay needed in default

you should make your own theme or a child them before making any changes or you could lose your changes on updates…

your own theme:  http://codex.simple-press.com/codex/themes/theme-basics/creating-a-theme/

child theme:  https://simple-press.com/simplepress-version-5-5-child-themes/ (new feature – codex coming)

easiest is child theme and then add this css to your child theme css file:

#spMainContainer {
    font-size: 100%;
}

just increase % or use fixed px number…