I've got a 512 running debian wheezy and my /tmp seems to be defaulting to 100m.

How can I go about resizing this?

The only place I can find it defined is in /etc/mtab, but I can't write to that file. (error writing mtab: Invalid argument)

it's not present in fstab, /etc/udev/rules.d or /dev/shm.

Thanks for any guidance!

in /etc/fstab

none /tmp tmpfs defaults,size=70% 0 0

then

mount -o remount /tmp

That did the trick, thank you!

Where are those tempfs mounts initially defined?