example_20cm_square.tex

\documentclass[showtrims]{memoir}

\usepackage{lipsum}  
\usepackage{xcolor}  
\usepackage[margin=0.5in]{geometry}

\setstocksize{20cm}{20cm} 
\settrimmedsize{20cm}{20cm}{*}
\addtolength{\textwidth}{-2cm}
\addtolength{\textheight}{-8cm}


\pagecolor{yellow}
\begin{document}

\chapter{Introduction}

\lipsum[1-150]

\chapter{Extra Pages}

\lipsum[1-150]

\chapter{Need That Long Document Length}

\lipsum[1-150]

\end{document}