Only one parameter and some basic math are required to set the size limit of an email message under Postfix.
To set the size limit of a message, edit /etc/postfix/main.cf and set message_size_limit=. The unit should be the number of bytes.
Now do some math: to allow sending a message of no more than 10MB, the number should be 1024*1024*10 bytes, which equals 10485760. Just fill this number to the other side of the equation.
Remember:
1KB = 1024Byte
1MB = 1024KB
10MB = 10240KB = 10*1024byte*1024byte
This is indeed confusing and I just showed how I suck at math. For detailed information, read this wikipage.