I don't like the look of trimmed marabou either, so I measure the marabou against the length of the hook before I tie it in. Another technique is to tie it in a little long with a couple of loose wraps, then grab the thick end and slowly pull on it until the tail looks the right length. I like the tail to be about the same length as the hook shank or maybe slightly longer.
Weighting can go all over the map, probably what would make the most sense is to consider the water you fish the most and weight for that. The bigger and faster the water, the more weight (within reason), the slower/thinner the water then maybe just a brass bead up front and no lead (or lead free) wraps. The other thing to consider is how you fish. Kelly Galloup fishes his streamers with no weight, but uses a full sinking line and a very short 36 inch leader. If you intend to fish with a floating line and a full leader than you will want to use some sort of weight on your streamers. Just realize that if you are putting a lot of weight on a fly it will be tough to turn over unless you really beef up your leader setup and step up in rod size to handle it. This is why you hear the term "chuck and duck" when it comes to streamer fishing with weighted hooks.
Chenille for body: This all depends on your hook size. The other day I tied up some wooly buggers on a size 6 hook and used medium sized chenille (I think that is what I was using), after a few wraps you will quickly see if it is too small or too big, I'd rather be on the small side than too large. YMMV!
Checkout this thread on marabou it will help with some of your questions.
understanding fly tying marabou