I'll bet few have given any thought to designing and sizing a Noah's Ark for specific animals. I like to consider the design or the style of an ark before I consider the size. Here's some "givens" for Noah's Arks that kids play with, as I have discovered them:
The size of the animals determines the ages of the kids that can play. The small animals are not suited for the very young. The small animals might get swallowed.
NOAH'S ARK THAT KIDS PLAY WITH
All the pairs of animals must be able to be stored within the ark.
Kids like lots of windows. Kids like lots of doors that open and close.
Ramps are fun for the kids to move the animals into/out of the ark.
Ark should not be top heavy, where it may fall over easily.
Less detail, that may get broken in heavy play.
NOAH'S ARKS PRIMARILY FOR DISPLAY
May be sized to fit on a shelf. May be narrower. Maybe taller.
Lots of windows and doors that open and close.
May have lots of detail.
As I work up a plan for an display ark for the smaller animals I'll probably add to the above lists..