Hi Dougie64. The Styles section of the sidebar can be opened with F11 (probably Fn+F11 if you have a laptop with the function keys re-assigned to non-important tasks)
If you have already changed the font size of the footnotes manually then we need to remove the direct formatting first as it will over-ride any Style changes. Ctrl+H, tick paragraph styles, select Footnote, Find All. In the menu click
Format > Clear direct formatting. The footnotes will revert to their old size.
Next we need to change the Style of the footnotes. Press F11 to open the Styles sidebar. Right-click on Footnotes (in paragraph styles, the leftmost icon under Styles ) and select Modify , click on the Font tab and change the size to 10pt, OK. All the footnotes current and future will be 10 pt. Cheers, Al