There tons of genera of cacti.
It just depends on the genus/species, but if you bought a cactus from a home improvement store and it was small it will grow to be large depending on the genus/species. Most of the cacti sold at home improvement stores are cacti that grow to be large.
You find out what genus it is in and then what species it is, there many good websites for cacti identification out there, I suggest using on of them.
No cacti store water, not milk. A genus of succulent called Euphorbia leak a milky latex when damaged, they look like cacti but are not. This is probably what you are thinking of. The latex produced by plants in the genus Euphorbia is a skin irritant and can cause temporary blindness. They are common in cacti collections, and are often sold as cacti.
Trichocereus is the genus of psychoactive cacti.
You should ask what are some different genera of cacti, because the genus name comes before the species name. An example would be Echinopsis (genus) oxygona (species), so that would look like Echinopsis oxygona.
"Any of several spiny, shrubby, or treelike cacti of the genus Opuntia having cylindrical stem segments." - American Heritage Dictionary
The Sahara Desert is not typically known for cacti, as its arid conditions are more suited to other types of succulents and xerophytes. However, some cacti species can be found in regions on the fringes of the Sahara, such as the Opuntia (prickly pear) and some species of the genus Echinopsis. These cacti have adapted to survive in harsh conditions, but they are relatively rare compared to the diverse flora that thrives in more hospitable desert environments.
Cacti of the genus Hylocereus are originally native to Mexico and grown around the world now and produce what is known as 'dragon fruit.'
Pachycereus pringlei (Cardón) It is the tallest cactus species in the world, with a maximum recorded height of 19.2 meters (62.9 feet) It lives in Baha California.
Yes: Prickly Pears, Barrel cacti, Saguaro cacti, Aztekium cacti, and Echinopsis cacti a.k.a the hedgehog cactus
cactus for a single plant cacti for plural
All cacti are succulents, but not all succulents are cacti.