Many email providers only accept email attachments to a specific maximum. The maximum size limitation is different for most providers and can range for a Megabyte to 20 or 25 Megabytes that are supported by Yahoo Mail and Gmail.
It is therefor important to have the means to display the size of attachments in an email client prior to sending the email to make sure that the email will not be rejected by the mail server due to size limitations.
Displaying email attachment sizes can also be helpful for users who want to reduce the size of their mailbox.
Attachment Sizes is an add-on for the Thunderbird email client that adds size information to all attachments in the web browser. The file sizes are displayed next to each file so that its usually easy to determine the overall size of all attachments. The total size of all attachments is displayed when hovering the mouse cursor over the attachment area in the email client.

This does however only work when sending emails, not when the email has already been received. But the total size is not nearly as important in that case.
Attachment Sizes is compatible with all Thunderbird 3.x clients. It can be downloaded from the Mozilla Thunderbird add-on repository.
Update: The Thunderbird add-on is no longer necessary, as Thunderbird 3 is now displaying attachment sizes by default.
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Do you mean the add on called Attachment Sizes version 0.0.5 from the webadres: https://addons.mozilla.org/en-US/thunderbird/addon/878
Sorry link added.
nice one …