The biggest culprits are unnecessary email in your Sent, Spam and Trash folders.
Which folders/messages in myemail.cox.net are using the most space?
I don't see a feature to itemize the size of each folder.
How can I see the amount of space being used by each message/attachment ?
If you click a folder, there is a Sort By option under View. If you sort by Size, the largest email will be at the top of the list.
How can I download messages/attachments to say a thumbdrive?
> Select the checkbox to the left of the email (click Select All)
> Click the icon with the 3 horizontal lines (More Options)
> Choose Save File As
...or you could just download its attachment(s)