3" by 5" notecard.

\documentclass[8pt]{article}

% This sets the page dimensions. If you prefer a vertically-
% -oriented card, swap the values of paperwidth and paperheight.
\usepackage[margin=0in, paperwidth=5in, paperheight=3in]{geometry}

% This package just produces some dummy text for you to play with.
\usepackage{lipsum}

% Set the paragraph skip (the vertical space between paragraphs)
\setlength{\parskip}{0in}

$ Set the indent of the first line of each paragraph
\setlength{\parindent}{0in}

% Set the line spacing (double, single, etc.)
% Note that '0' leaves no space between lines, allowing for
% the maximum workable area.
\linespread{0}

\begin{document}
\lipsum[1-2] % makes dummy text
\vspace{.9cm} % explicitly creates vertical space
Hopefully this will help some of you out.
\end{document} % woo!!