Hi Andreas,
You can do this in your php.ini file. Look for the following settings:
post_max_size
upload_max_filesize
To edit the php.ini file in linux, use the following command:
# vim /etc/php5/apache2/php.ini
If your php.ini file is not located in/etc/php5/apache2, run either:
# locate php.ini
or
# find /etc/ -name php.ini
which will locate the file for you.
Once you make these changes, restart apache and check the php info page in Administration -> General Settings -> Server -> Server Info
To restart apache:
# sudo service apache2 restart
or
# sudo /etc/init.d/apache2 restart
Regards,
Adam