"How long is the perfect knife for fighting?" -- Not an issue. Weight, flex, perceived comfort and security of the grip, etc., will matter more than length. Try a bunch of different blades out and see what you like.
"How long does the blade have to be to reach and penitrate organs?" -- One inch.
"What are those organs (best to hit)?" -- Internal ones. I'd recommend you not limit your targets to specifics, but focus on what targets of opportunity an opponent would present: more than likely, arteries. Check our archives here for a few different threads on various topics.
"Straght blade, or curved or tanto, serations or not?" -- Absolutely. See my first answer: it will be up to you. Because basically anything you can hold in one hand can be used effectively, these are mere details. I have both straight and curved blades, clip, drop, and tanto points, etc., serrated and non-serrated: and they all work the same way at this level. Again, try a few out, and see what feels right. Never buy on looks alone.
"What is the minimum length?" -- 1" or more. And if you're familiar with swords, you'll quickly discover there is no maximum length for a knife--you can easily sail right into the world of swords without finding that limit.
Good luck.