Something as big as a destroyer or frigate would be a major undertaking and the size of a game table. On the otherhand, Something akin to an MTB or E-Boat would probably be, scale wise, about the width of a baneblade and twice as long. Whilst not as heavily gunned as a destroyer or frigate, they could still be realitively well armed with cannons, heavy machine guns, torpedos etc
You scout cars are looking good, i was thinking of using something similar (maybe a bren carrier) for my guard army.