Altering font size using label expression?
Yes! One can definitely set different font sizes for different fields within a single label. Please see this Link: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00s800000027000000.htm
Look at the font attribute. You can set a particular size for the important part of the label and a smaller size for the less important part.
I should be reading the help more often...
Yes I can: "<FNT name='Arial' size='18'>" & [LABELFIELD] & "</FNT>"