they can get to 6" but in aquarium 4" is more common. I think you could keep them in a 25gallon but you need open spaces for them to swim in so barely any decor in that size.
I have 3 in my 57G tank. They were all about 2.5" when i bought them, now they are all 3.5 - 4.5". I havent seen them bigger than 5-6" in an aquarium or LFS.