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, 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.