I had one problem with file attachment larger than 10MB. Users couldn't send it although I have setup squirrelmail (SM) to be able to attach files summed up more than 20MB and I had modified php settings as per here. The problem was not in SM setting. It was postfix. By default, attachment size that can be sent by postfix is 10MB ~ 10240000 byte. How did I know it? I looked in log file (for my system it is in /var/log/mail/errors. For other system, the file to look is /var/log/maillog). The line looked like this:
Feb 26 16:30:53 webmail postfix/sendmail: fatal: firstname.lastname@example.org(74): Message file too big
message_size_limit = 20480000
This sets limit to 20MB.
Some other parameters you need to change are in file php.ini which is usually located in dir /etc. Set their parameters as above or higher values as below:
post_max_size = 20M
upload_max_filesize = 20M
reload or restart postfix when you're done:
service postfix reload
service postfix restart
There's also mailbox_size_limit directive. You need to change this if SM can not open mailbox sized more than 10 MB.