This looks great on you, and definitely keep it long. I would try the next size up just to "see" - not saying this one's not the right size, but if the next size up is just a teensy bit looser, while still being form-fitting, it might be even better. In the meantime - I think this dress is veering into "killer" territory. The purple looks amazing on you, and the style is fabulous.
Angie was right when she described this dress as being something that can add curves AND subtract them. I didn't quite get this when I read it, but now that I've seen it on a more rectangular/IT-ish person, I can see that once again - she was spot-on.
One of the things I really like about this dress is the fact that it seems like something you could wear from September to about May or thereabouts.
It looks great on you, and no - don't shorten it.