You have to retrieve the mail message using GetMailMessage() method of your Imap or Pop3 client object (see the following tutorial http://www.rebex.net/mail.net/tutorial-imap.aspx#downloading for more information).

The MailMessage class has a property Attachments with collection of message's attachemnts. Each Attachment has a GetContentLength() method, returning length of the attachment in bytes.

Here is a complete sample how to retrieve size of all attachment of individual mail message on IMAP server:

   // create client, connect and log in   
   Imap client = new Imap();
   client.Connect("imap.example.com");
   client.Login("username", "password");

   // select working folder
   client.SelectFolder("Inbox");

   // get the topmost MailMessage in the folder
   MailMessage message = client.GetMailMessage(client.CurrentFolder.TotalMessageCount);

   // show count of message attachments    
   Console.WriteLine("Subject: {0} [{1} attachaments]", message.Subject, message.Attachments.Count);

   // show name and size of each attachment 
   foreach (Attachment attachment in message.Attachments)
         Console.WriteLine("\t{0} ({1}KB)", attachment.FileName, attachment.GetContentLength() / 1024);