I glanced down at the bottom on my gmail account. My mailbox is 30% full, almost 3G. While I have a very long way to go before the account become 100% full, the organized part of me just want to clean things up!
If you want to delete very old emails in GMail, use this search filter:
is:unread before:2011/01/01
This will return all mail that is unread and older than Jan 1st of 2011. I actually add in additional filter, as I organize most of my emails by label. So since I follow a large email list call "python", I can select and delete all the emails from that list by adding label:python to my search criteria. Noting that I created a automatic filter to label all emails from that mailing list with the label "python".