The Large interactive checklist script lets you do just that.
Add it in, and the checklists shown on your closed cards will get a font increase.
If you like, you can choose a different font size by clicking the /scripts/large-interactive-checklist/large-interactive-checklist.css link in our Github repository, and editing the px font-size value, and copy-pasting the CSS directly under the CSS slot in the board’s Developer Tools window.
In case you’d rather increase all of the board elements at once, consider giving the board zoom a try!