[metapost] How to convert .mp file into jpg file?

[liuqiang]

I use "mpost" to generate eps file. 
My target is encapsulate jpg-styled images into my Emacs Muse files.
So, next steps are followed:
1. mpost somefile.mp
2. generate one tex file:
+--------------------------------------------------
\documentclass{article}
\usepackage{graphics}
\begin{document}
\begin{figure}[h]
  \centering
  \includegraphics{somefile.1}
\end{figure}
\end{document}
+--------------------------------------------------
3. pdftoppm it.(convert pdf into images)
4. convert ppm style into jpg file.

After fininished these steps, I found the jpg file was as large as one A4 paper.
I know the default size of pdf is A4.

I don't want this.
Can anyone tell me how to get jpg-styled images as large as originial eps images?

If I use "mptopdf" directly, the command fails:
+--------------------------------------------------
! Undefined control sequence.
l.44 \documentclass
                   {article}
?
+--------------------------------------------------

Thank you.

Btw:
my tex distribution: TexLive 2007.