If it's a data validation dropdown list, you can't change the font size.
There are a couple of workarounds here:
http://www.contextures.com/xlDataVal08.html#Font
If it's a combobox from the Control toolbox, you can change the font
size by switching to Design view, opening the Properties window, and
changing the Font properties.
If it's a combobox from the Forms toolbar, you can't change the font size.
