A work around, which may be tedious, depending on the volume of text you're working with is to set font-size in HTML tags to em(s) using inline CSS. For instance <p style="font-size: 1.5em;">Your Text</p>. You can perform this from the HTML editor. Rather than attending to each line of text or paragraph individually, you can manage it on larger scale using <div> tags, and you can wrap your whole, or sections of the page in said <div> tags.
In case you're new to em(s), the example of 1.5em means 1.5 x the default font. So if Canvas' default font size is 10pt, 1.5em would display as 15pt text. Lets say you set the font size in your Rich Content Editor to something other than the Canvas default, 14pt, you can also apply the em to it as well. (e.g. 14pt x 1.5em would display at 21pt font.)
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