Ask your admin to set the Max File Size in your course properties to 4194304 (the maximum bytes allowed)
Ask your admin to check the php.ini file (usually in /usr/local/lib/) to be sure the the upload_max_size variable is not set too small. The default is 2MB, but it doesn't hurt to increase this by few MB.
Have you been able to upload other smaller files? If not, be sure that the access permissions on the ATutor "content" directory are set to world writable, or is owned by the web server user, often called "nobody" (your admin will know what I'm talking about).
Also make sure you don't have any illegal files in uploaded zip archives (see IllegalExtentions in the ATutor configuration options). The admin can temporarily change these settings globally if you wish to upload scripts or executable files.
http://www.atutor.ca/atutor/docs/configuration.php
There's also a variable called "MaxFileSize" in the ATutor configuration options that can be increased to set the default upload size allowable to something larger.
Let us know what happens with the above. If you're still having problems, send us the url to a phpinfo() page on your server so we can see its configuration.
http://ca.php.net/manual/en/function.phpinfo.php