I don't know about once only. You could change your /etc/postfix/main.cf then restart postfix let the mail go through then change the main.cf back and restart again. Complicated but Thats all I can think of. Anyone else?
This is at the bottom of my main.cf The comments should help you understand it
#Limit users to message and attachments to 20M
message_size_limit = 20971520
#Limit mailbox size to 100M
mailbox_size_limit = 104857600
Also If you are using squirrelmail i got this from http://www.hughesjr.com/content/view/15/
Also in /etc/php.ini is the variable upload_max_filesize ... it is normally set to 2M (that is 2 mb) ... set this to the max size for your attachments....I'll assume 20M
upload_max_filesize = 20M
post_max_size = 20M
memory_limit = 20M
(you may need to adjust the post_max_size and memory_limit to be higher than 20M ... maybe as high as 40M to get 20M attachments.