If it was me I'd go for as big as my house would allow, although if it's only for a maximum of an hour then I guess you could go smaller. There is absolutely no way I'd leave Billy in the one he has now which is 32" as he can't lie down fully stretched, and this is how he likes to sleep. It's ok for the car as our journeys are rarely more than an hour. However Billy is a fully grown adult, a pup won't need as much room as this at first.