I noticed some common confusion in the comments posted... TmpFS is not necessarily kept in RAM, it uses virtual memory and can thus be swapped out to disk as well...Which is good IMHO. (Not everything you cache may not be used that often.) So the maximum possible size is not limited by the amount of RAM you have, but by the virtual memory size.
Actually, I tent to use the size of my swap as the maximum size for tmpfs on /tmp, since I'm hardly using my swapfile for anything but suspend to disk.
(If you want to keep as much data in memory, you may want to lower the "swappiness" value in /proc/sys/vm/swappiness anyway)