Description:
dvipng has a -T option to force render dimensions.
It ignores the height and produces a square based on the width.

Additional info:
release texlive-bin 2009.5-5
build http://tug.org/svn/texlive/trunk/Master/bin/x86_64-linux/

Steps to reproduce:
Get your favorite tex file.
Render dvi with latex.
dvipng -T "4in,8in" -D 100 foo.dvi
Examine images created; all are 400px by 400px.