This question already has an answer here:
It is Spring and I decided to clean and reorder a GMail account with thousands of mails inside.
I would like to have a list of all senders, is it possible?
I can't think of a way to do this from within Gmail. If you're up for using Thunderbird to accomplish it, this video should help you figure out how to get you what you're looking for: http://www.youtube.com/watch?v=k3pV2wdShDg