You could use the javascript window.open()
function in a pure HTML button. Copying from here, you can see an example defining the popup window size etc.
<a href="#" onClick="window.open("http://www.w3schools.com","_blank","toolbar=yes, scrollbars=yes, resizable=yes, top=500, left=500, width=400, height=400");return false">Link text</a>
Another alternative would be to use jQuery and specifically the overlay tool.
Hope I helped!