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Platies were originally put into the genus of Platypoecilus, which was abbreviated to "Platy". By the time platies were reclassified into the Xiphophorus genus, "platy" was commonly accepted as the common name for the species. That "platy" is easier to pronounce reliably that Xiphophorus (or abbreviations), helps maintain the "platy" name.
Individual fish may vary in their final length, but most platies stay under 4 inches in length (excluding caudal/tail fin). Size is impacted by environment, including tank size and quality, with optimally cared for fish having most chance to reach their genetic maximum length.
Technically speaking, individual platies can survive in tanks as small as one gallon. However, like most animals, they do best when they are provided more than just the minimum. Juveniles and adults should be provided a baseline of 3-4 gallons per fish. If you intend to breed platies, or acquire females which have been exposed to males, you should account at least 10 gallons for fry.
Similarly sized fish with low aggression and some physical resistance make good tank mates for platies. Common neighbors include guppies, sharks (aquarium varieties, not the ocean predators), and small catfish species. Additionally, snails and platies will generally leave each other alone. As the combinations of fish that live in similar water are quite overwhelming, your best bet is to do research on each type of fish you're interested.