If I have understood the problem correctly.
On my Linux-Mint system, I can view and set the font selection and size for a spreadsheet, like in Writer it is on the Formatting toolbar. Select VIEW > TOOLBARS > FORMATTING. This switches the formatting toolbar on and off. Then adjust the font as with Writer.
You can access the Spreadsheet Cell Styles to change the default font settings by F11 > STYLES AND FORMATTING > CELL STYLES > DEFAULT > MODIFY. Then adjust the fonts to suit for the default and heading styles…Peter