How do I the comments on YouTube large enough to read? They were until suddenly Fridsy

Try doing this to see if it helps: Go into Safari Preferences > Advanced tab > Accessibility --Never use font sizes smaller than ___

I have mine set to 18, but I think it maxes out at 24pt.

Alternatively, you can also use keyboard function pressing Option key with Command key and the + key at the same time. This adjusts text size on the current webpage.