You can change the font size (via Settings > Customize Toolbars), but it affects the whole toolbar. If it only has style tabs then that might be OK.
I'm guessing you use some of the styles more than others. If so, you might want to create buttons for the ones you use most often, and leave the rest off the toolbar entirely. If you're using the default Opus 11 or 12 toolbars, the full list of styles should still be in the Lister menu, near the top-right.
To create a button for an individual style, the command is like this:
prefs style="commander"
Where commander is the name of the style you want.