How can I filter out large emails?

 In PEP you can test the size of an email using "bytes".

EXAMPLE: lets say your on a dial-up connection and you don't want large emails,  you can add a rule in your mailrule that in this example bounces back email over 1.5MB

bounce if bytes > 1572864 "Your email is over 1.5MB in size, I am on a slow connection and don't accept large emails. Please sent text only."

Here is a handy javascript based byte converter...:

1 Byte = 8 Bit
1 Kilobyte = 1024 Bytes
1 Megabyte = 1048576 Bytes
1 Gigabyte = 1073741824 Bytes

The Byte Converter by Malte Philipp