<a href="pop.cfm" title="Open in New window" onclick="window.open
(this.href, 'popupwindow', 'width=300,height=200,top=50,
menubar=0,taskbar=0'); return false;">
*all code should be on one line
This should be it, although i typed it quickly from memory and I haven't tested it.
Put in the position (top, left) you want, or take it out.
I don't think you need px for measurements, but I'll soon be shouted down if I'm wrong.