changing size of the site name

This is kind of minor, but I wanted to reposition and resize the #site-name, and encountered this CSS in page.css:

/* h1 is for the front page only, all others use the div. */
#branding h1 {
  font-size: 1em; /* reset h1, do not change! */
} 

Uh, so why shouldn't I change this font-size? I don't seem to be able to change the font-size elsewhere in the #site-name-related CSS.