Lazaros
There are not really any right or wrong answers to those questions, only points of view. I will give mine!
1. As a
very broad rule of thumb, one third of the length of the jig. For very long jigs though, less than that. Usually about one third of the length mean the hook point will be somewhere close to behind the eye - that's good as many predator fish strike in that area.
2. The width of the hook should preferably be wider than the width of the jig.
3. The perceived wisdom is that short shanked hooks are usually those that work best for jigging. Again though that is only a guideline. Personally I have a lot of success working with a very long shanked hook (Owner Jobu), but that is not typical of the hook types used for jigging.
4. I would urge you to learn to tie your own assist rigs (it is VERY easy) and not buy the made up assist hooks. This will save you money, give you a greater sense of achievement, and allows you to determine exactly how you want to present your hook. I make mine with kevlar assist line (usually Varivas Assist in the 260 lb size), a tube of super glue and some heat shrink. There is a very well written beginner's guide to jigging which includes how to tie the assists, here:
http://www.360tuna.com/forum/f68/intro-jigging-gman-reference-beginers-4449/Leaving aside my peculiar taste in Owner Jobus for jigging
, my favourite heavy jigging hooks are Owner SJ41 and Owner SJ51 in the largest sizes. Sadly, it seems Owner have discontinued the SJ41 in size 13/0 but you can still find them around. I also sometimes use Owner Gorilla in the large sizes.