Dunster is correct about the various meanings of the word "cot." When I make baby quilts, I think in terms of the size of the baby mattress, and I subtract 6"-8" from the length. That said, the more powerful determinant of the finished quilt size has to do with the pattern I'm making.
There's a nice discussion of baby quilt sizes here:
https://www.quiltinaday.com/communit...=2&topic=22202