Yes, you can easily do this, and it shows how flexible Fastmail can be.
- First you need to decide if you want to view the conversation sizes or the individual message sizes. Then go to Settings>Preferences>Conversations and make your choice.
- If you use Folders mode (also on the same Settings screen immediately above the Conversation selection), in the Search Mail box above the message list enter in:* and the Enter key. If you then sort by size, you will see every message in all folders. You can also search for certain messages in all folders by using in:* followed by a space character and then any normal search string, and they can be listed as described before by size.
- If you use Labels mode, it's easier. Just select the All mail label in the label tree at the left and of course choose Sort by size. This will show messages in all folders except Spam and Trash. To add Spam and Trash to the labels shown in All mail, you need to enter in:* into the Search Mail box.
The advantage of using the built-in Fastmail web interface features over IMAPsize is speed and ease of use. I have over 200,000 messages in my Fastmail main account in many folders, and I can very easily see all of them sorted by size and delete them as needed. IMAPsize was useful in the past before Fastmail added many new search features.
Bill