Coverage annotation with a $SIZE pseudoitem setting of 0 will allow dynamic size adjustments in ArcMap.
Arcedit: EDIT <covername> ANNO.<subclass>
Arcedit: SELECT <appropriate select statement>
Arcedit: CALC $SIZE = 0
Annotation with multiple symbol numbers will appear in the Symbol number list. To change the size for just one symbol, click the Text Symbol button and change the size in the Symbol Selector dialog box > Options section.