Hello, and thank you for your question.
If you are using a desktop mail client, then you should be able to click the Size column to sort your email messages by their size. If you don't see the Size column heading, try right-clicking on the bar where your other mail headings are, then place a check mark beside Size to have that column available.
If you're logging into your email account via webmail, you can use either RoundCube, or Horde, which both have the Size column available for sorting your messages.
Please let us know if you had any further questions at all.
- Jacob