One possibility - in Firefox you can go to the Tools menu, select Options, then the Content tab, and in the Fonts & Colors is the Advanced button (on the right of the first line) - click on that.
In the pop-up that you then see you can set what fonts are used and their sizes, AND there is a tick box for "allow pages to choose their own fonts, instead of my selection above". If you make sure that is unticked then the font sizes that you have selected will be used.