Thanks for the update. You may get help in the text size by having a look at this website (I stumbled across it by accident): https://www.w3schools.com/howto/default.asp
Let me know how you get on.
I've tried all combinations of Font-Size: 'nnpt', 'nnpx', 'xx-large', 'larger', '150%',
Nothing seems to work!
font-size: 50px; (or whatever size you like) should work fine - I just tried it in the web developer tools in Firefox and the size changed as expected.
If you're adding it to a custom css file you may find it doesn't update straight away as the file can be cached. Maybe try it and be patient to see if it changes after a while.
(though you could instead put the custom CSS into the theme settings where any changes will appear instantly when you reload any page)
I was adding it into custom CSS in the theme settings, the other changes did change on re-load (colour/font).
Will give it another shot.