On a Plesk server, the maximum size for an individual e-mail sent through qmail is unlimited. You can limit this size by adding a number to the /var/qmail/control/databytes file.
If you wanted to limit this to something like 10MB, you can just run the following command:
echo "10485760" > /var/qmail/control/databytes
This will limit the size of messages (including attachments) to 10MB as a maximum.