mount -o remount,size=5G /tmp/

Siehe https://wiki.archlinux.org/index.php/Tmpfs

By default, a tmpfs partition has its maximum size set to half of the available RAM, however it is possible to overrule this value. To explicitly set a maximum size, in this example to override the default /tmp mount, use the size mount option:

/etc/fstab

tmpfs   /tmp         tmpfs   nodev,nosuid,size=2G          0  0

To specify a more secure mounting, specify the following mount option:

/etc/fstab

tmpfs   /www/cache    tmpfs  rw,size=1G,nr_inodes=5k,noexec,nodev,nosuid,uid=user,gid=group,mode=1700 0 0

See the mount(8) man page and Security#File systems for more information.

Reboot for the changes to take effect. Note that although it may be tempting to simply run mount -a to make the changes effective immediately, this will make any files currently residing in these directories inaccessible (this is especially problematic for running programs with lockfiles, for example). However, if all of them are empty, it should be safe to run mount -a instead of rebooting (or mount them individually).