Customising the LaTeX output of gutenmark

I'm attempting to customize the LaTeX output I get from the GutenMark application for use on a BeBook One (6" screen, 170dpi). I've managed to get the margins looking reasonable, but the body text is simply too small to read comfortably. I was wondering whether there were any classes available for the production of Large Print books, that I might use?

Here's what gets produced by default:

\documentclass{book}
\usepackage{newcent}
\usepackage{geometry}
\geometry{verbose,paperwidth=5.5in,paperheight=8.5in,tmargin=0.75in,bmargin=0.75in,lmargin=1in,rmargin=1in}
\begin{document}
\sloppy
\evensidemargin = -0.25in
\oddsidemargin = 0.25in
\mag=1000
\raggedbottom
\newcommand{\mdsh}[1]{\mbox{#1}\linebreak[1]}
\pagestyle{empty}
\newcommand{\nodate}{\date{}}\nodate
\title{The Turn of the Screw}
\author{}
\maketitle

-

\documentclass[14pt]{extbook}