yybbaa,

You can't do it without a third-party program. I use MailWasher Pro ($37) which displays a list of messages on your ISP's server (including the size of each) and allows you to choose to bounce them (if they are spam), delete them (which you would do in the case of a huge message), or accept them. The program is available as a 30-day full featured test. You can download it from here.

You might also want to check with your ISP and see if they have a Webmail option for viewing/retrieving mail. Waiting messages are usually displayed in list form, and many of them include the size of the individual messages. Most allow you to designate individual messages for deletion prior to downloading those you wish to keep.

One other free option would be to use a utility called ePrompter, which is normally used to let you know when you have messages waiting in a particular account without having to open your POP3 e-mail client to check. While ePrompter doesn't display individual message sizes, it allows you to view information about the sender/subject, and if there are a bunch from unknown or spam senders, delete them from the server ahead of time. You can also preview the messages in plain text format, which should offer some insight as to whether they include attachments.

Thank you both for the information. As it turned out it was a series of photographs and I just waited it out and eventually it downloaded.  The sender told me later that she had received a message to wait for several hours before attempting to send anymore messages. 

 I will check with my isp about this issue and if no help there will try the programs you suggested.