How do I set the default size of /dev/shm?

I'd like to change the default size of /dev/shm in Lubuntu 11.10 (also known as /run/shm now, I guess).

It doesn't seem to appear in my fstab:

# / was on /dev/sdb5 during installation
UUID=66ac63f0-45fa-4766-9d20-7c56bcd0460d /               ext3    noatime,errors=remount-ro 0       1
# /home was on /dev/sdb7 during installation
UUID=227f1b29-5d04-4c28-9c9c-ea70b1726434 /home           ext3    noatime         0       2
# swap was on /dev/sdb6 during installation
#UUID=9e13b7cc-1f75-4b4e-9e79-c0f7368de353 none            swap    sw              0       0
/dev/mapper/cryptswap1 none swap sw 0 0

tmpfs /tmp tmpfs defaults,noexec,nosuid 0 0
tmpfs /var/tmp tmpfs defaults,noexec,nosuid 0 0

Try putting a /dev/shm line in the fstab file to override the default settings:

tmpfs /dev/shm tmpfs defaults,noexec,nosuid,size=100M 0 0

If that doesn't work, you can try remounting it after it's been mounted at boot, for example by putting this line into the /etc/rc.local file

mount -o remount,size=100M /run/shm