So for example, let's say you have a link:
<a href="http://example.com" id="external-link">Link to example.com</a>
You could add a click-handler to the link that calls window.open()
e.g. of the window.open call
//I'm assuming here that you've defined "theHREF" to be the href of the "external-link" in the click handler function.
window.open(theHREF, "NewWindowDescriptiveName", "width=800,height=600");
There are a bunch of other options you can set alongside width and height, check out the MDN documentation