No.
Size is completely random. There is a recent thread about the probabilities.
http://kittycats.biz/forum/showthread.php?tid=16308
From the data there, I'd put the odds at a bit better than 1-in-83 (1.2%).
Sorry, but the best you can do to "breed for a Megapuss" is hope you get one, on average, from every 84 boxes you produce.
