How big should the crate be?

[jazzy]

My little fella needs a new crate. He's been using a cat carrier *face palm* which he still likes to sit in. But he's a growing boy, and though he is comfortable now, I feel like it's going to be too cramped for him.

What size should I aim for?

He's a Shiba, 3 month old.

We were looking at this one:

http://www.petco.com/product/10316/P...atesandKennels

but can't decide between the 300 and 400 series.