I think the next size up will be too big--the dress already looks like it's the right size in the shoulders and is appropriately low in the chest.
After reading all the comments and looking at your pics, I'm a bit torn. Like a previous poster, my opinions are coloured by the price tag and for the kind of garment that price is getting. Somehow, the dress looks too casual to my eye to cost so much, though I would understand the price tag for a formal gown, structured piece, coat, etc. If it cost, say $200-300, I would jump at the chance. The colour and flow are beautiful, and are a nice change from more structured sheath silhouettes, though. But I wouldn't purchase it if I didn't plan on wearing it primarily as is--no leggings, no toppers.
Janet's distinction about loving the idea of the dress vs wearing it is spot on. If you have the patience/time, maybe hang on to the dress and revisit it in a month.