Mine maxed out around 12" to 14", much larger than that is pushing it. They are well over three years old now, almost four. If you are wanting a larger pike, go with Crenicichla cincta. They get quite large, and are thick to boot.
Tankmates work out best if grown up with the pike. You think of it, it can work. If it won't work, the pikes have a way of letting you know. Large enough would be within 3", much smaller or larger than that and you can have issues quickly. Although I do have a huge Red Tailed Giant Gourami with mine that is sort of a peace keeper between the pikes. LoL
BTW Mark, still love that tank. I shalt do that again someday, with bare bottom this time. You are correct that with gravel its a pain. Hahaha