Carp have fairly big mouths so you can get away with all sorts of lengths, as mentioned have a try and find a length that makes you feel comfortable that you hook them well. Look to find a length that starts hooking them properly in the mouth and you find you just lift into fish much easier and get good hook holds. Try the round hooks, they seem to work so well. I whip the line a fair way down the hook to hang the bait under the hook with a gap between them, I find movement around the bait will make it waft around in the water making it easier to spot.
I usually have my hook around 5-10mm from the bait, whatever bait i use. Another tip i use is to use a black marker and colour in the hook but wait till its dry before using it, it takes away the glint if your in shallow water.