Depending on your account type, your email's size may not exceed the following limits:

Free Edition: 100,000 Byte
Professional Edition: 100,000 Byte - if you need to send larger emails, please contact support.
Enterprise Edition: Unlimited

Your email's total size is summed over the email header (Subject, To-Line, ..), email message (plain text and HTML), personalization and attachments.

If your email is too large, this is most probably due to a large attachment. We advice to upload large attachments to a webserver first and to simply include a link in your email. If your task can only be completed with attachments, you can also use file compression  (for example WinZIP) to reduce your email's size.