If you want to know the size of any messages in Gmail as seen in any other email services, you may not be able to get it.
Gmail does not offer this feature and it seems like there are not working on it too.
You can see the attachment size but not the message size.
More often we never want to see the size of the message but we all worried only about the attachment size.
If you want to send inline images you may have to worry about your message size which you cannot see.
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I was facing a problem quite similar to yours, I almost ran out of gmail space and thought about giving up and buying more space because there was no comfortable way of sorting my mails but then I got across this site
http://www.searchgmailbysize.com/
It allows you to sort your mails by size. Oh, and it's free and online. what more can you ask for?