When we got Lincoln at 9 weeks we started with the 24 inch length which he quickly outgrew. Now he is 8 months @ 40 pounds & 22 inches tall and we got the 36inch long crate and it's just right.
For puppies you get two choices - start with a small crate and buy larger replacements as they grow, or find a crate which can be divided into smaller areas while your puppy is little. This post on our main site might help you decide which is right for you
Sarah has it, there. Personally, because I like to have little puppies by my bed for a week or more, I prefer to have a small 'baby puppy' sized crate there. At this time I also have a larger more medium-sized crate which fits under the kitchen table - since the pup will spend a lot of time in the kitchen with me until toilet trained and integrated with the other dogs. Then, once they are too big for the crate under the table, they graduate to the rest of the house and get a giant-sized crate in the office - where the dogs are when I'm out. They will remain in that one until through adolescence and all annoying behaviours have passed! Really - at some point, you will need a giant-sized crate during adolescence and smaller crates are pretty reasonably priced, so it works out to get various sizes and that way be more flexible with their positioning when using the smaller ones with a younger puppy - the giant one will only fit in a few places around your house.
Many thanks for the comprehensive reply and apologies for the late reply. First time I have logged on since posting.