You don't have that capability with data-validation drop downs. One
work-around is to set up a drop-down control that moves around on the sheet
to where you need it and you can control things such as the font size in it.
Second attempt ... sys error, so don't know if this will be a repeat or not.
Can't adjust font size in data-validation drop downs. But you can use a
drop-down from the controls toolbox (View | Toolbars | Control Toolbox) and
the detailed instructions from this page: http://www.contextures.com/xlDataVal10.html