Hop Studios have published a post that hopes to help us all prove that ExpressionEngine can handle what we throw at it (hosting permitting). They've done this by listing out what they perceive to be the largest ExpressionEngine sites they know of, and boy are there some big ones! They get phenomenal amounts of traffic.
Continue reading: The 10 Largest ExpressionEngine sites on the Hop Studios website.
If you have any other questions about ExpressionEngine that you would like answering to help you make the decision to choose it as your CMS of choice, please either ask in the comments or send me an email via the 'Contact' section.