I've used text messaging for years and always reach for my reading glasses, a small source of irritation, never once thinking that I could simply make the messages larger by increasing the font size. Until I saw large letters on someone else's phone. Dope! Homer Simpson moment.
My Android phone allows me to change just the font size for the Messaging app, which is all I really want. iOS devices can also increase their font size, but it applies across email and some other apps, which I wouldn't care for.
The procedure for changing font size varies by device, so best to Google it or just explore your phone to find it.