Hey @SouthPoleCat, you can adjust the all the font sizes for the entire app by going to View > Zoom In.
Or, you can adjust them using CSS. One way to do this is to add the following to userstyle.css (rendered markdown) and/or userchrome.css (the rest of the app). You can add those here Joplin > Preferences > Appearances > Show Advanced Settings:
*{
font-size: 30px !important
}
This code will also affect things like icon sizes. Once you start tweaking CSS, you can have a lot of fine-grained control. Join us over in Share your CSS to see a bunch of ways people have customized things.
Hope that helps, feel free to take a screenshot if there is a particular area you want to make larger.