Receipt font sizes are controlled via the Management Utility. To change the font sizes, first close Librid and then open Management Utility. In Management Utility, find section Receipt under the Librid3 branch:

Footer print font size is the size of the footer font, at the end of the receipt.

Header print font size is the size of the header font, at the top of the receipt.

Text print font size is the size of the main text of the receipt.

Change these to what you want them to be, and then click on the Save button at the bottom left. The changes will take effect when Librid 3 is next started.