If you're talking about the tiny date-time display, it looks like you could do it by modifying the ConfigEdit.xml file (search around here for instructions.) It looks like the font size is configurable there, but not in ProntoEdit itself. Look for the and tags -- a font tag is embedded in them, currently set to 8-point standard font (STANDARD 8).

Haven't tried this myself, but based on other reported tweaks to the XML file it seems like it should work.