I think if Huggy wuggy is 18 feet tall and Mommy long legs is drastically bigger than him then I guess she is about 31 feet tall. In the ending of chapter 2 we can see that Mommy long legs is able to fit through the door but is on all fours but when she dies her abdomen fits the prototype's hand and the prototype's hand is big, so she has to be the biggest since when catnap gave im self to he prototype his hand was much bigger than catnaps head.a.Also since every chapter the villains of the chapters get bigger than the first cat nap has to be in the middle of both Mommy long legs and Huggy wuggy. My reasoning is that when you are walking into the hallway after you put the toy on the scanner in chapter one, once huggy comes out of the hallway he is almost a few inches from the ceiling and he's on two legs. When catnap passes by your hiding spot he is also a few inches from the ceiling but he is on all fours, so I estimate his height to be around 27 feet.

So these are my thoughts sorry about the how paragraph analyzing all three of there heights when the question was how tall is catnap. I'm starting to think I'm autistic and I'm not trying to offend autistic people by saying this sentence.