Congrats on the new addition.

Midwest has awesome crates now and I think other brands have them too. They come with a divider panel that allows to adjust the size as the puppy grows.
crates

I'm not exactly sure what size would work as he's mix & you just don't for sure how big he will be when he's full grown but I'd guess that the 42 L x 28 W x 31 H" would work just fine.