Good questions. it would make sense that Space docks are scale 15 or 16. It would take a lot of damage to completely take one down. Maybe we can come up with a homegrown rule that calculates how many deaths happen during certain damage rolls that affect Structure; or complications related to entire sections being compromised.
Maybe a roll to determine location of a hit (unless called shot at higher difficulty) and then roll to kill? This way it could be possible to blow open the docking area of a spacedock and cause minimal damage to habitat area.