Duncan,

The font size is set in the step 4 advanced settings, styles and models, then page item "page text"

This is used as a standard for your whole site.

If you wish a different one in your dynamic content then you can change it here, but you need to manualy change the text for website pages made in x5 later on because at start of text object in a page it takes over the same settings.

Already made pages are not changed.

Let me now if you need more info.