When pet crates and pet carriers were relatively new pet owners would just get a product with the thought that it is the end all be all of the expense for a pet dog. Before getting any pet accessory or furniture, take your pet's  breed , height standard and its weight. Years of handling dogs and being around pet  and hunting dog owners there have been a number who have pinched pennies which result in a dog's suffering. Seeing  hunting dogs crammed into their old dog crates does more bad than good, too bad others take awhile to understand.
The proper way to fit a dog would be to follow these considerations:

Lying space- measure your pet as it is lying down on its side with its legs stretched to where it can.
Height    -  try out possible pet carriers and see how much head room your pet will have when  it gets in. Is your pet stooped? The best carrier or crate is one that is comfortable.

When buying without your pet, it is best to opt for the larger size. When you buy for your pup, it is best to buy using the standard size your pup's breed can get. Some pet stores also have divider panels available which you can install in the adult sized crate or carrier until your pup grows into it.