I can confirm that it is huge (and luxurious) inside but I can't really prove it. I've only seen the inside once and Snoopy was in there sipping a martini in a smart burgundy robe but I can't remember the episode.
Snoopy's kennel is supposed to be big inside. From what i remember, it has a cellar in which he keeps root beer. He has parties in it and when all his relations come to stay, they all go in the kennel, so it must be huge (like the tardis). I'm almost certain you never see inside the kennel though, you just see people bringing things in and out of it.