I have the following code which opens up a popup window on Click of a button on my site

<input type="button" Value="Click here to open the Tag Board" ONCLICK="window.open('test.html', 'Sample', 'toolbar=yes,location=no,directories=no,status=no, menubar=no,scrollbars=yes,resizable=yes,copyhistor y=yes,width=750,height=600')">

How can I change it so that the window appears maximised?

just say

window.open(theURL, '', 'fullscreen=yes, scrollbars=auto');

thats it.