I've seen a number of emails with reduced widths--down to 300 px width--I suspect this is a reflection of the increasing number of messages that are being consumed on smartphones. Even a 600px wide email looks tiny when viewed on a mobile device, while a 300px message is small but readable.
Given that every mailing list or database is unique, it probably makes sense to do some a/b testing with your database using different size emails and landing pages.