The default seems to be A4. Various ways to change this to letter:
Directly in the document (if converting to pdf) (not tested)
\usepackage{ifpdf} \ifpdf \setlength{\pdfpagewidth}{8.5in} \setlength{\pdfpageheight}{11in} \else \fi
In Unix, for all documents (not tested)
sudo texconfig-sys paper letter
In Windows, for all documents
With TeXlive, we can use texlive manager (run as administrator) and choose letter for all documents, under the configuration tab.