Strange issue regarding my.cnf

Question

I'm getting a weird message from phpmyadmin saying:

#2006 - MySQL server has gone away

What I found online is that I should increase the size of max_allowed_packet witch should be in the my.conf file witch is located at /etc/my.cnf. Untill now everything is fine, but the problem is that this file does not contain what I need.

Here is the content of the .cnf file on my server:

[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
user=mysql
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0

[mysqld_safe]
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid

Where should I look in order to increase the size of the packet? I'm new in the linux world and I'm lost.