twentythirteen h1 and h2 not resizing responsively

You would need to access your style sheet (style.css). Your site uses media queries to target specific screen sizes and apply different rules when the stylesheet interacts with them.

In section 4.1, it defines the site title as:

.site-title {
    font-size: 60px;
} 

In Section 8.0, just beneath @media (max-width: 643px) it defines the site title as:

.site-title {
    font-size: 30px;
} 

In other words, when the screen size is less than or equal to 643px wide, the site title adjusts to 30px.

In your case, you’d want your Section 8 include a rule for h2 (exchanging the # for the desired number of pixels:

.site-title {
    font-size: 30px;
}

h2 {
    font-size: #px;
}