how do I change the h2 font size?

I think you can change the size with this added to the END of the child style.css:

h2 { font-size: 28px !important; }

Adjust the 28px as needed.

You may find that you need to increase the line-height to get the proper spacing.

This will affect every h2 on the page. Your theme distinguishes among h2 in various locations so you could adjust each of them separately. For the page heading:

.type-page h2 { font-size: 28px; }

For the numbered entries:

.entry h2 { font-size: 14px; line-height: 1.1em; }

For comment headings:

.comment h2 { font-size: 18px; }

Just add font-weight to the style:

.entry h2 {
   font-size: 14px;
   line-height: 1.1em;
   font-weight: bold;
}