Boas and Carpets are great!
If you're thinking towards Aussies, I really enjoy my Carpets a lot. They will make you up your game as a snake handler. They are very determined where they want to go and faster than BPs and Boas. They are major climbers which make them one of the best display snakes.

Esmarelda, Jungle Carpet
The Irian Jayas are the smallest, averaging out at 5 ft or a little more. Jungles average 6-7 ft. Coastals 7-9 ft are the largest.

Etta, Coastal Carpet

Lizzy, Coastal Carpet
Now a big thing you run into is that there are a lot of intergrades, mixing, making lots of mutts out there. That's not bad, mutts are beautiful. But if a guy sells you a Jungle and you are only expecting 7 ft max, and it has Coastal in it, it may get bigger. Not only longer, but thicker as well.
There are "pure" breeders like Nick Mutton, just go in with both eyes open.
If you do settle on a Carpet, I highly suggest this book as the Bible for Carpets, it's not cheap, but worth every penny. And tons of photos too!

Esmarelda, the art lover

Mack, Coastal Carpet