Oh well... right now, the font is derived from the "default GUI font" (which is usually the dialog font).
I have added some new avast4.ini items that make it possible to control the font size (they'll be present in the next avast! update).
In particular, they are:
MsgFontSize - for the usual messages (that's what you are looking for)
InfoFontSize - for the Info screens
InfoTitleFontSize - for the Info screens titles
All the values should be in the [AAVM] section; if not present, the default values will be used.