The genus that modern Komodo dragons belong to, Varanus (which also includes other species of monitor lizards) is thought to have evolved about 40 million years ago. "Ancestral Komodo dragon" might refer to some other genus in the varanid family, or the broader varanoid superfamily (including the Gila monster) which as discussed here did have some members alive in the Late Cretaceous, such as Estesia. Although not as large as the one seen in Terra Nova, the late Cretaceous varanoid Palaeosaniwa is thought to have reached a length of five to six feet, smaller than the modern Komodo dragon which reaches a length of about ten feet.
The largest known fossil varanoid was the prehistoric Komodo dragon Megalania, or Varanus priscus. It is only known to have existed fairly recently in the Pleistocene epoch, between 26,000 and 19,000 years ago according to this page. Fossils are incomplete, but comparisons with modern Komodo dragons suggest it might have reached lengths of about 23 feet.