My dog, when a puppy, quickly grew out of her initial carrier. I got a large crate which was given to me from an owner of a large dog. Cassie does very well in this size and honestly a smaller one wouldnt be good for her. Its about 3 feet long by 2 feet high.
Given the expense I would opt for a large one so you dont have to keep buying larger ones.
Pictures of the crate and with her in it. She likes to go in it on her own thus the door is open