Indus civilization stamp seals are usually square to rectangular, and about 2-3 centimeters on a side, although there are larger and smaller ones. They were carved using bronze or flint tools, and they generally include an animal representation and a handful of glyphs.
Animals represented on the seals are mostly, interestingly enough, unicorns—basically, a bull with one horn, whether they're "unicorns" in the mythical sense or not is vigorously debated. There are also (in descending order of frequency) short-horned bulls, zebus, rhinoceroses, goat-antelope mixtures, bull-antelope mixtures, tigers, buffaloes, hares, elephants and goats.
Some question has arisen about whether these were seals at all—there are very few sealings (the impressed clay) which have been discovered. That's definitely different from the Mesopotamian model, where the seals were clearly used as accounting devices: archaeologists have found rooms with hundreds of clay sealings all stacked and ready for counting. Further, the Indus seals don't show a lot of use-wear, compared to Mesopotamian versions. That may mean that it wasn't the seal's impression in clay that was important, but rather the seal itself that was meaningful.
Sources and Further Information
Rao, Rajesh P. N., et al. 2009 Entropic Evidence for Linguistic Structure in the Indus Script. Science Express 23 April 2009
Guide to the Indus Civilization
Bibliography of the Indus Civilization
Study of the Indus Script at Harappa.com includes an article by Asko Parpola, essential reading to understanding this issue.
Steve Farmer, Richard Sproat, and Michael Witzel. 2004. The Collapse of the Indus-Script Thesis: The Myth of a Literate Harappan Civilization. EJVS 11-2: 19-57. Free pdf to download
