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

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