I don’t know how to change font size in my glitch domain.
Changing the font in Glitch is like web development in general - you use CSS. You can look at existing examples, remix them, adjust them, and see what effect that has.
So in ~hello-express style.css line 47, the font-size inside the button is set to 16px; if you remix ~hello-express and then edit public/style.css line 47 to instead say 72px, you will see the text inside the button gets bigger.
Hope this helps,