The Volm Mothership (name speculative) that landed in Boston was massive in size even compared to the Espheni 'Motherships' (which are probably more accurately called 'towers' or 'defense towers'. When it landed it clearly occupied a sizable portion of Boston, probobly making a realistic description "city-sized". Though the specifics are unknown it appears that the Volm considered it to contain enough force in and of itself to liberate Earth from the present invasion. Given the Espheni's minimalistic philsophy that might be more realistic than it appears on the surface. Early speculation was that it was an example of a Volm Generational Ship, but comments made by Cochise in Season 4 (Ghost in the Machine ) seem to indicate that the Volm 'Broodmates' and young were hiding in a distant star cluster, therefore making it likely the Volm ship at Boston was purely as war ship.