Allow me to provide some clarifications about changing the font size of your checks in QuickBooks Online.
The steps given by my peer, Angelyn_T is applicable for QuickBooks Desktop version. Based on the screenshot that you've provided, I can see that you're currently using QuickBooks Online. That said, changing the checks' font size in this platform isn't available. You can only make changes to the sales forms fonts by customizing it.
However, our developers are constantly working to improve the product and appreciates the help of users in bringing things like this to their attention. I'd encourage you to visit our QuickBooks Online Blog to be updated about our latest news, features, and updates including what our Product Care Team is working on.
The Community always has your back and I am here to help you anytime. Please feel free to post an update by leaving a comment below. Have a good one!