Our dog van boxes are three across and two rows high across the back of the transit. they can just stand up or lie full length or donut but the width is not excessive.
One dog to a cage though an unexpected increase in numbers has one pair in a central cage and one traveling loose as she cant do the back steps.
I like them singly in case of being thrown about in an accident and also if i am away i can leave them treats, toys or drink and know who has what. No arguments while traveling or anything like that. They always have the same cage, they get to like their own space and some of mine will ask to go in to their room when they feel like a lie down on the vet bed instead of the nasty cold ground.

the cages are long, about 3 ft 6 inches as i like to be able to leave bowls etc in with them and not have them knocking them over, just less than two foot wide and high.
Doors each end.
fold up shelf means they can jump in and out easily.
The floor of the van is the floor of the cage and the wheel arches are within the side cages but there is still plenty of room because of the length.