Just a quick paintover and edit. Mine isn't exactly perfect but you get the idea

It's purely an anatomical and balance edit so the pose suffers quite a bit, just a matter of rejiggering until you get both right.

The outfit is quite nice but it's draped over bad structure, always make sure what is under the clothes is solid otherwise your picture will never look quite right. As far as shortening the arm, there isn't really a way to do it without erasing everything around it. Never be afraid of doing so though, big changes like that are going to have more impact to the final image than spending an extra hour getting the AA right or something.
EDIT: Maybe it just clicked for me what pose you were trying to achieve, another very quick drawing to illustrate a point.

Basically if you're trying to achieve a sense of movement and balance you need pay very close attention to the line of action in the pose, make sure everything kind of follows a single path. To keep the pose nice on this I've lifted her left foot off the ground so rather than trying to balance on it awkwardly she's using it to assist her (try leaning your upper body to the side and see what your legs do), then it's just a matter of trying to balance the amount of mass on one side with the other. Because she's assumed to be in movement there is some leeway as her momentum to her right will counteract the extra mass.