Here’s how you’ll get to it. Open Mail, of course, and then click the gear icon in the lower-left corner. Choose “Get Account Info” from the menu that’ll appear.
In the subsequent window, you’ll see a drop-down menu at the top. By default, the account that’ll be chosen will be from whatever server-side mailbox you had selected in the sidebar before clicking the gear icon. I find it’s easier to just choose the correct one from this dialog box, so swap that drop-down to the account you’d like to get the sizes for, then select the “Quota Limits” tab (or “Messages on Server” if you’re using Exchange).
Depending on how much email you’ve got in the chosen account, you may need to go get a cup of coffee. Or lunch. The oh-so-familiar spinning gear will keep you company while you wait.
After the process is done, though, you’ll get a neat list of the mailboxes associated with that account and their sizes. You can click the headers at the top of the list to sort by name, size, or number of messages, too.
I’m pretty proud of my Inbox being at zero.
I’ve arranged my list by size here (which you can tell because of the arrow within that column header), but I can also see that I’ve got more than 3,000 sent messages stored on the server. That’s not a ton, I guess, but I still feel so untidy. Anyhow, another cool piece of info available is at the top of that last screenshot—Mail attempts to figure out how much available space you have on the server for the selected account, which can be handy if you’re using a smaller email provider that severely limits the amount of storage you’ve got.
Still doing A-OK with this one.
When you’re done perusing your first account, go ahead and look at all the rest of ’em by using that top drop-down menu. And after you’ve seen which mailboxes desperately need archiving, check out this tip I wrote last year on doing just that. If you’d prefer, you could also just follow the first part (about creating “On My Mac” mailboxes) to remove the offending messages from your mail server but not from the Mail program. But then you can feel all accomplished for getting things sorted out and cleaned up! I’ll be proud of you, I promise.