<a href="javascript://" onclick="_open( '/forum', 300 , 300 );">
Ссылка
</a>
<script type="text/javascript">
function _open( url, width, height ) {
window.open( url, '', 'width=' + width + ',height=' + height + ',left=' + ((window.innerWidth - width)/2) + ',top=' + ((window.innerHeight - height)/2) );
}
</script>