Never heard of a scissor tail tetra. If you mean The Scissortail, Rasbora trilineata, they can grow to about 8"/200mm but they very rarely do, 4-5"/100-130mm is more usual, under good conditions. Very peaceful, even big ones won't bother other fish.
i have nine scissortails and they are great fish. when i sit infront of my tank i'm not sure if i'm watching them or they are watching me lol. i'm thinking i might have to get rid of them though coz my tank is too crowded. I'll miss them