I am assuming that you've tried the Control+ keyboard sequence or the View / Text Size / Increase menu item without any relief (i.e. it doesn't make the typeface bigger but pads the characters with more spaces instead).

Try setting the menu option Edit -> Preferences -> Content -> Fonts & Colours (Advanced) and unselect the option Allow pages to select their own fonts .... You can then try and manipulate the choice of fonts and sizes in the section above.