Now click the Modify button below, and you can select a new font and font size under formatting. In case you didn't or it hasn't, you'll have to scroll through the 'Select a style to edit' menu, find it and select. As long as you've highlighted your footnote text before visiting this menu as we noted in the last paragraph, Word will automatically have selected Footnote Text (Hide Until Used). Go to the bottom of the Styles box which has popped up, and click the third icon from the left which is Manage Styles. This brings up the full Styles menu, which grants access to the hidden bits including footnote editing.
Now go back to the Home tab on the top menu, and under Styles in the Ribbon, at the bottom right there's a tiny little button you can click (see the screenshot at the top of this article) or you can use the keyboard shortcut which is Alt+Ctrl+Shift+S. To change the font of the footnote, first of all highlight your footnote text with the cursor.
