Do you want to do this for all links automatically (and if so, for internal links inside your own site as well?), or selectively for specific links?
Selectively for specific links, without resizing, would be accomplished by adding (in HTML mode, in the WP editor) target="_blank" to the link
HTML Code:
<a href="http://www.otherdomain.com/" target="_blank">...</a>
Selectively, with resizing, replace the whole link with
HTML Code:
<a href="javascript:newwindow()">Click Here!</a>
and at Theme Options -> HTML/CSS Inserts -> CSS Insert add:
HTML Code:
<script type="text/javascript">
<!--
function newwindow() {
window.open('http://wordpress.bytesforall.com/','something','width=300,height=200,resizable=yes');
}
//-->
</script>
Firefox will not obey to the size settings though, if it set to open links in a new tab instead of a new window.