Normally people say to avoid any fish half the fish's length or smaller, but that breaks down a bit on eel-like fish. The shape of the fish you're keeping with it is more important than their length. Long torpedo like fish (such as danios) are going to fit in it's mouth more easily than tall fish (such as Red Blue Colombian tetras).
As a general rule, I'd stick to taller fish that are 5"+ long to avoid tempting an adult Bichir. It doesn't matter if the fish can actually fit in it's mouth, it matters what the Bichir thinks will fit. Plenty of them have choked on fish that were too big to swallow.