I design my ships with:
Room, 1 wall, 1 empty, 1 wall, Room
and
1 Ceiling
3..5 Room
1 Floor
3 Engineer-deck
1 Ceiling
3..5 Room
1 Floor
... repeat
sum= 13..17 meters|blocks per 2 main decks + 1 engineer deck.
with
50% of the engineer-decks filled.
some rooms filled which may take 2+...