<a href="http://support.microsoft.com/default.aspx?scid=kb;EN-US;q224526" target="_blank">http://support.microsoft.com/default.aspx?scid=kb;EN-US;q224526</a>

From this article, it sounds like if you create the partition using NT4 setup, it will make it 4GB, but if using a 3rd party (partition magic) you should be able to use 8GB of system partition.