In FreeBSD, "md" devices are "memory disks" aka, RAM disks. Their content will go away and be lost if you reboot or re-create them. There isn't a way to resize them per se, because their content is transient. Instead you just re-create them at the size you want. The size is specified using the `-s` flag to mdconfig or in fstab (see the [fstab man page][1], under the `EXAMPLES` section, there is a reference to an md10).

I should point out to you that while having `/tmp` as md is fine, having `/var` as md is probably a bad idea. While the data under `/var` is "variable" by definition, it is not transient. If you make `/var` a md you will lose this data after you reboot. For example your pkg database is stored there, as well as many other important files. Making `/var` a md will mean that, for example, if you reboot you will lose all your pkg info and be unable to `pkg upgrade` as there will be no record of what's installed.


 [1]: https://www.freebsd.org/cgi/man.cgi?query=fstab%285%29