I hate to say this but Outlook actually works best for me for keeping my email account to a sensible size. I use a simple VB script that I trigger with a keyboard shortcut which strips all attachments from a mail but adds the filenames of the attachments stripped at the top of the mail ("Attachment(s) removed: , , ..."). Very useful to keeping track of what was actually sent.

The other things that Outlook does really well is converting pesky HTML mails to plain text. I don't mean to just display them as plain text, but actually convert them to plain text, scrubbing all that superfluous HTML nonsense. Just open a mail, edit, format as plain text, save. I would love to find another (FLOSS) client that can do this as easily/efficiently.

I keep all old mails (that are not spam or otherwise superfluous) in a folder in my account (not Gmail, but the same principle) and can always quickly refer back to them. 100,000s of mails over 18 years barely make a dent in my allowance -- just about a GB out of 10 GB. At that rate, I won't run out of space with this workflow before I kick the bucket.