Average animal size was about that of a housecat and no need to bring on animals who lived in seawater. Genesis 7:1-5 breaks the types of animals down between 'clean' and 'unclean' and only 7 of one type and a pair of the other. This isn't to be taken literally, but many do.
If this is not to be taken literal, Why should other stuff in the bible be taken literal?