The Luna theme has a "quickfit" plugin that adjusts the header font size with an inline style rule, based on your screen size (mostly to make it look better on mobile devices). You should be able to add this to the bottom of your CSS and specify whatever font size you'd like:
#site_header header #store_name {
font-size:36px !important;
}
Otherwise you could consider uploading a header logo in Customize Design > Customize which will replace your header text.