Hi Violet,
ok let’s try to fix it step by step.
If you have firebug installed and active, you’ll notice an arrow from the tools available. pick the arrow to select an element, and select one title of your site with that firebug arrow; firebug gives you some css details on the style tab panel.
You’ll notice it highlights something like
.entry-header .entry-title {
font-size: 1.57143rem;
}
wich is the css property actually set for the page titles.
within firebug you can add properties and see the result in realtime.
this is the big thing about Firebug, and the reason why WPyogi suggested it.
then you just have to add the new property to your child theme stylesheet, and you can do it as you prefer via ftp or with WP files editor.
Let me know if you’re in doubt, I’ll be glad to help you as I can.