Edit Font Detailswindows 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 over every font that Webster uses. You can change the size and aspect ratio as well as the font name and weight.
You just select the HTML style that you want to change from the list on the left and then pick the Font and Size that looks best to you.