I really think some people are idiots. Have you see the SIZE of that ***** 🙂 Everyone is complaining she is too small around the web. OMFG That is just her ****in head FFS. If you look towards the top left of her, you can see one of her back legs and OMFG it is like a tree trunk. Not to mention we have yet to see her "shakin that ass" and that is gonna be J-Lo stylee, I guarantee it.
By looking at the picture, (thanks Turin for the size comparison) it seems to me that Shelob is indeed smaller than what i expected her to be. Remember, Frodo is a hobbit (i.e. small) Shelobs body may just be a little over as tall as a man. That wouldn't be too frightening. Wargs would be scarier in packs.
I agree with Exa in saying that Peter jackson would never show Shelob in trailers... i hope. : )