Font size in the dropdown list must be the same size as in the combo box, so
if you change the combo box's font size at the right "time", you can get the
larger font. Unfortunately, there is not a unique event that occurs when you
dropdown the combo box, so perhaps you can use the GotFocus event of the
combo box to make the font size larger, and then use the LostFocus event to
return the font to its normal size.
Examples, assuming that the large font size is 20, and the normal size is
12:
Private Sub ComboBoxName_GotFocus()
Me.ComboBoxName.FontSize = 20
End Sub
Private Sub ComboBoxName_LostFocus()
Me.ComboBoxName.FontSize = 12
End Sub