Joplin does nearly everything I need and offers a good set of preference settings BUT there is no preference to adjust the text size. It's too small for my needs.
There was so e discussion about this and some workarounds offered 18 months ago. I feel this should have been addressed in the course of general development and updating.
UI difficulties are a big barrier at first point of contact, to wider adoption of Joplin as a viable alternative to more prominent (often expensive and green) notes applications.
-- Ian Greig
There are a number of ways to adjust the text size. You can do a global zoom via the menus or ctrl + +/-, the editor font size via the appearance options or you can adjust pretty much anything via css.
What exactly are you trying to adjust?
Just a guess, but they may be talking about the mobile app, which doesn't allow any text size modifications (while also not supporting any userstyles). It does follow the system font size though, but it may not be enough, at least in some cases. There's a reason why some apps allow to tweak font size separately (e.g. WhatsApp, Samsung's SMS app, etc.).