The problem [if it is one] for 40K is that canon is not fixed. Take Space Marines; in the early days their armour was not only for protection but contained auto-injectors to administer the drugs necessary to keep their super-human physiology from tearing itself apart, now they can go without armour for long periods and suffer no ill effects.
As to BFG I always took my size / crew figures from the books Execution Hour and Shadow Point (Cruiser: about 3.3ish km long and 12 - 14,000 crew); now the RT corebook has given us the 5km cruiser & 95,000 crew.
Which is canon? - Both of the them; each source is every bit as authoritative as the other.
The real question is which suits your version of W40K better? Choose and go with it!
DW