In the past I had good success with using HTML and mm units in CSS to position and size elements, but I've used a browser to print the HTML.

Thanks Corion

I'd overlooked mm in CSS...I shall give that a go.

Using a browser to print the labels is actually preferable here. The data is accessed via a browser so using PDF or SVG formats would mean downloading a file and printing that, which is, of course, an extra step.