Gmail gives you 10GB of storage space to work with, but sometimes even that's not enough. Find Big Mail (free for any @gmail.com address) is here to help. Just enter your email address on the Web service's home page, grant it access to your inbox and watch as it identifies which emails take up the most space. (It works in any Web browser.)
Once the service is finished scanning your inbox, you'll receive a detailed summary email complete with easy-to-read space breakdowns and links to the 20 largest emails hogging your megabytes. The service also lists the emails by size in Gmail's sidebar for later deletion.