What size would be best for him?

I have a mini Australian Shepherd border collie mix who will be 5 months this week. He is almost 30 pounds already and I need to buy him a new crate. The one he has is I believe a 30 inch although I meant to get the 36 (I wasn’t paying attention to what I was doing). I was wondering what size would be best for him since I plan on using the crate for awhile even after he’s fully trained. The pets stores near me only carry the 36, 42, and 54 inch ones. I just want something that he can sit comfortably in but not something where he’ll have too much room. Plus it also has to be long enough for when he sleeps because he likes to sprawl out when he lays down.