What to look for in a “survival knife/dagger”? Length? Material? Shape? Something else?

I want to start a knife collection. Someone gave me a WWII era knife to start out.

Out of curiosity, what makes the best combat/survival knife?

Is a longer knife always better? I've seen some with a 12 inch blade? Or is that bad?

I've seen straight knives with sharp pointy edges, I've also seen weird curvy knives. Which is better?