Provided you can keep the water quality up to par, an 11" cube should be fine for a leaf fish. They don't move around a whole lot and grow no larger than 3", so they're quite comfortable even in very small tanks. Just make sure there's plenty of structure so it doesn't get bored.
The problem with leaf scorpions, though, is that they almost never eat dead food. I've heard some people claim to have trained theirs to eat frozen or dried shrimp, but I've never been able to despite many attempts. If you want to keep one, make sure you have a ready source for feeder shrimp.
Leaf scorpions are really easy to keep and adjust to tank life almost instantly. I've never had one that didn't eat (live food of course) and behave normally from the moment it was caught.