I'm trying (and failing) to use the An shadow feature on a simple vector arrow I've created in AI. Importing in the An using a variety of setting, the shadow always appears around the arrow's document edges rather than around the more complex shape of the arrow.
You're right, the box-shadow feature (which is what the "Shadow" feature does) will put a shadow based on the image's real dimensions. However, you might want to check out the shadow feature under "Filters," which actually does do an appropriate drop shadow on images with transparency. Unfortunately, CSS Filters are not supported in all browsers. You can get a little more information CSS Filters here:
Hope that helps,