Weird. It should be under Gameplay, but the one you are showing is shorter than the version I am seeing. When I click the settings cog, below Auto-scroll, there's a Base Font Size and then Font Family settings.
I think I see the issue, are you using the browser version? Because in the browser version I see the option to increase font size, but on the downloadable version for Windows I don't.
I was using the downloadable version, so I can play when my internet is down too. I'm guessing that maybe the Browser version is newer than the versions available for download then.