Of the three known species of elephant, the African Forest Elephant is the smallest at 6 feet at the shoulder. The Asian Elephant comes in at 8 feet, with the African Savannah Elephant about 10 feet. Males come in somewhat larger.
Among the cryptids the African Pygmy Elephant is said to be 3 feet tall, while the Short Trunked Elephant (also African) is described as being the size of a hippo, and about the same shape.
In extinct elephants the size varies from 3 feet tall at the shoulder for the Sicilian Pygmy Elephant, the Santa Catalina Island Pygmy Mammoth, and the Wrangel Island Pygmy Mammoth to 15 feet at the shoulder for the Imperial Mammoth of the Southern Great Plains of the U.S.
Keep in mind that elephants are one of the few mammals that keeps growing all through its life. A seventy year old matriarch will often be as much as 2 or 3 feet taller than her 40 year old daughter. Which means it would be possible for your adventurers to meet an 18 or 19 foot tall Imperial Mammoth (if your campaign world had Imperial Mammoths, that is).
Keep in mind that elephants are intelligent, have amazing memories, and have been known to learn the local human language to some extent. And don't forget their ability to communicate using infrasound. Which can travel for miles. So if the group wipes out a herd for the ivory, they may find themselves facing a second herd out for revenge, or find that every elephant for miles around has taken off for safer parts.
Finally. If the group finds themselves in a part of their world where elephants have not been extensively hunted, they may find literally tons of ivory lying on the ground. Ivory is an amazingly durable substance. The early European explorers in West Africa sometimes reported walking for long distances on piles of elephant tusks, without once seeing the ground.
