Quick Font Setupwindow looks as follows. (Note The colours are much better in real life. But if you're already using an Acorn Machine then they should be correct)
Using this window you have control the basic fonts that Webster uses. The standard font is the basis for e.g. Normal, Quoted and Emphasised text. The fixed width font is used for styles like Preformatted, sample and Code Fragments.
You just select the base Font and Size, individual HTML styles etc. will be scaled up or down as required. You can change the details of the fonts if required using the Detailed Font Setup Window.