Little help here...
Just got another LG V20 H910 (AT&T), my second one.
For some reason, on this particular phone unit, I CANNOT CHANGE THE FONT SIZE.
I follow the steps, get to the font size option, but no matter what size I select (large, extra large, huge), the font size just stays the same.
Everything works fine on the same model phone I already had for a while, just not on this new one. Weird, to say the least...
Are you talking about the size for the phone itself, or in a specific app?
If you are talking for the phone as a whole, that option in "Settings" should work. You may, however, have to restart the phone first and then make the change.
Now, if you are trying to change the font size in a specific app, or make it show for a specific app after changing it on the phone as a whole, that may be where the problem lies. Not all apps allow for the change of text size. So no matter what you set the phone's text size to, the app may well have a fixed text size.