Molly fish get to around 4.5 inches in length. In the wild, molly fish tend to grow bigger than mollies that live in captivity.
The size of your molly fish will also depend on their species. While common mollies are generally only 3–4.5 inches long, other molly fish species are smaller or even bigger.
For example, balloon mollies are tiny and only grow to be 2.5–3 inches long. Black sailfin mollies, however, are bigger than other molly species and can grow to be 6.75–7 inches in length.