root@mrfloris:~# cat /etc/php5/apache2/php.ini |grep "apc.shm_size"
#apc.shm_size = 128
root@mrfloris:~# locate apc.ini
/etc/php5/conf.d/apc.ini
root@mrfloris:~# cat /etc/php5/conf.d/apc.ini |grep "apc.shm_size"
root@mrfloris:~#
That's all I could think of.
The # in front of the line means it's commented out, so it's not actually set to 128 for my system. So it goes to 'default'; whichever that is.
Make a new file called phpinfo.php with content <?=phpinfo(); and run it from the browser.
Search for APC, maybe it will disclose more details. And if not, then type
locate apc.php
cp /path/to/apc.php $HOME/public_html/apc.php
and run apc.php from the browser.