Welcome back to World of Warcraft.
If you use the default User Interface the font size is changed by right-clicking on General chat and then clicking on “Font Size”, however, that’s only for the chat channel.
The guides now detect other addons that control fonts and retrospectively use that setting from said addon.
Addons include SharedMedia or better yet, ElvUI. With ElvUI you can change the font + font size. There are several video guides out there on how to customize ElvUI and there’s a video guide on how to make your hook (addon) to add in your favorite fonts to run and merge with ElvUI’s addons. It’s not overly complicated, he does step through with what you need to do.
Video guides I’d recommend are:~
ElvUI Installation
If you need any assistance I’ll be happy to help out