How much information is included in the databases of the different search engines? How do these numbers strike you?
Google 53 Billion Pages
Yahoo! 8.4 Billion Pages
MSN 3.7 Billion Pages
Those are estimates from four researchers at the Stanford University Dept. of Computer Science, who have come up with a method of Estimating the Index Sizes of Search Engines (the article has been removed from the Stanford pages – see comments below for more details) based only upon information that could be gathered from the public interfaces of the search engines.
There are a number of questions about the results that they received, but they consider and include some discussion about potential errors that may throw off their numbers.