First there is need to check in the exim configuration what is maximum upload size allowed.
In order to determine you may use the following command
exim -bP | grep message_size_limit
in order to change it
vi /etc/exim.variables.conf
message_size_limit=50M


In order to increase the file size in SquirrelMail navigate to
cd /var/www/html/
vi .htaccess


Forsquirrelmail the defuault upload size is 5M and in order to increase it just edit the following file.

vi .htaccess
php_value upload_max_filesize 30M
php_value post_max_size 30M