You can't change the font size for a Data Validation dropdown list.
However, you can use programming to adjust the zoom setting when the
cell is selected. There's some sample code he

http://www.contextures.com/xlDataVal08.html#Larger

Trond wrote:
I have a validation drop-down list in a cell and since my
sheet is big, I've zoomed it to 50%. When I want to select
an Item from the list the font is so small that I can
hardly read it. Can I change the font in the drop-down
list?

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Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html