It means that you need a field in the cgi for 'data_size'.
I'd suggest adding (iirc):

<INPUT TYPE="HIDDEN" NAME="data_size" VALUE=100>
[download]

in your form (could be wrong there).
Basically, you're looking for a URL that looks something like:
http://www.myserver.net/cgi-bin/speed_test.cgi?data_size=100
You could always hardcode the data size though, depending on what suits you.