There may be some confusion between tmpfs(5) and md(4)-based /tmp, set up by the tmpmfs and tmpsize directives in rc.conf.
tmpmfs is the original method of creating a fixed size memory disk, then mounting it as /tmp.
tmpfs is the newer, experimental virtual-memory based method.
The only thing required to set up tmpfs is a line in fstab (as the kernel module will be loaded automatically).
You don't need to specify a filesystem size for tmpfs; it simply uses as much virtual memory as required.
tmpmfs is the original method of creating a fixed size memory disk, then mounting it as /tmp.
tmpfs is the newer, experimental virtual-memory based method.
The only thing required to set up tmpfs is a line in fstab (as the kernel module will be loaded automatically).
Code:
tmpfs /tmp tmpfs rw,mode=01777 0 0
You don't need to specify a filesystem size for tmpfs; it simply uses as much virtual memory as required.