To open in a new windows with dimensions and everything, you will need to call a JavaScript function, as target=”_blank” won’t let you adjust sizes. An example would be:
<a href="http://www.facebook.com/sharer" onclick="window.open(this.href, 'mywin',
'left=20,top=20,width=500,height=500,toolbar=1,resizable=0'); return false;" >Share this</a>