Changing the paper size

The template uses the KOMA documentclass scrartcl. You can switch to letter paper by replacing the first line with

\documentclass[letterpaper]{scrartcl}

You will probably have to play around with the layout as it is optimized for a4 pages.

Carsten's suggestion is good, just to add some other possible ways of changing page parameters.

A very quick and dirty hack, just after the \begin{document} add:

  \pdfpageheight 8.5in 
  \pdfpagewidth 11in

For a different way - to get finer control use the geometry package (just change the parameters to suit)

\documentclass{scrartcl}    % classe article di KOMA
\usepackage[total={6.5in,8.75in},top=1.2in,
  left=0.9in, includefoot]{geometry}