Would some dinosaur egg be the most massive cell that ever existed? Are dinosaur egg relatively small compared to eggs of modern birds?
The biggest known egg was laid by Aepyornis, the so-called Madagascan "elephant bird." See this short video. Of course, the eggs of most dinosaurs are unknown, so there could be something more massive waiting to be discovered. But the eggs of species much bigger than Aepyornis have been discovered, and the eggs just don't measure up to Madagascar's champion.

I'm not an expert on dinosaur eggs, but if you're looking for information, try searching for Protoceratops and Maiasaura peeblesorum. Protoceratops is a ceratopsian, ancestral to the more familiar Triceratops, whose remains are abundant in Mongolia. I believe many Protoceratops eggs have been discovered. (See At Last, A True Protoceratops Nest)

Maiasaura is a duck-billed dinosaur (hadrosaur) that left numerous fossils in what is now Montana. The fossils include some fabulous remains of nurseries (e.g. Egg Mountain). (See Maiasaura)

These links may not feature any information about egg size, but they should at least give you some background information. I'm sure Dr. John Horner has commented on the size of Maiasaura eggs somewhere. He's written on a wide variety of topics, including dinosaur growth rates, physiology, etc.

P.S. Here's a cool link. There's a photo of a fossilized Maiasaura embryo emerging from its egg (#4, I think). Another page (8?) claims that no dinosaur egg could be more than two feet in diameter.