Recently, I tried clearing my mailbox and was trying to sort the mails by size but I couldn't find the filter to do that. Went searching briefly online. There doesn't seem to be such an option in Gmail at the moment. I tried to send in my suggestion for that this morning but I couldn't find the link to do that either. *SHY!*
Now I found it and would like to share it with everyone that comes by. You can submit suggestions at this link: http://mail.google.com/support/bin/request.py
There are already a list of common suggestions where you can just click on the "Suggest it" button. Sorting by size is one of them. There's also an option to suggest other things, too, under "I have another idea" right at the bottom.