Another standard feature that's missing in Opera's built-in mail client is support for the IMAP4 QUOTA extension.
In some mail clients like Thunderbird, if your IMAP server account has a storage limit (and most do) and it supports QUOTA, you can check the properties of an IMAP folder to see how much space your messages are taking and what the limit is. In Thunderbird's case, each IMAP folder reports the total space used for the account and not the IMAP folder in question (because they all share the same quota root of "").
Opera doesn't support this at all so you can't tell how much of your quota you've used up. You have to rely on your mail provider's webmail if there is one.