Big draft mules are usually out of Percheron or Belgian mares (although other breeds are often used as well) and a mammoth jack.  To get a little smaller but fancier show type draft mule, the ideal jack to use on a draft mare would be just around 14 - 14:2 hands, which would add a little refinement to the mule.

Todd Bergland and Cynthia Kelly with their Draft Mule Team