You can tweak the size of the titles in the welcome section for mobile devices by editing the style.css file.

Go to Appearance > Editor and select style.css.

Then go to the bottom, you will see the following code.

.hero h1 {
font-size: 60px !important;
line-height: 60px !important;
.hero h2 {
font-size: 45px !important;
line-height: 45px !important;

You can change the number you see (60 or 45) to adjust to your needs.

In the next release of the theme we will be providing and easier solution for setting the font size on mobile devices.