While the control panel "font size" setting lets you change the overall font sizes used in the document, to get fine control over a specific part of the document, you need to create a tiddler, tagged with "$:/tags/Stylesheet", that contains the CSS rules(s) you want.
For your purposes, something like this:
.tc-edit-texteditor { font-size:150%; }
will make the font size for editing larger, while leaving the other fonts unaffected.
Q.E.D.
enjoy,
-e
Eric Shulman
TiddlyTools / ELS Design Studios
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