I don't know how much help this will be, but here goes anyway.
The clothes the characters wear seem largely inspired by folk costumes. While I believe that there are similar costumes around the world, what I've seen most are the Scandinavian ones. In Sweden this clothing is known as folkdr�kt (litteraly folk/people dress) or allmogedr�kt (allmoge being a word for peasantry) since originally these clothes were worn by the peasant population. I've heard it being called "Bunad" in Norway, but I'm not sure if that term is representative of the entire spectrum of clothing.
The modern designs of these clothes are mostly made up in a time when national romanticism was running wild, or so I've heard. These days they're used as ceremonial or festive clothing, and they are actually an acceptable option in certain ceremonial dress codes.
As the movie takes place in the vicinity of a fjord and Norway is known for this geography I found it likely that they'd stick to this theme with the clothing as well. Swedish variants of this dress that I've seen personally varies a lot in color pallete, but the Norwegian almost always feature black as a primary color, which Anna's dress does.
Maybe there are patterns for Bunad dresses out there? The interest in these clothes is alive and well here and there after all. It might be a little difficult to understand for someone not fluent in the nordic languages though.
On the case of the width of the skirt, there's a scene in which Anna tries and fails to climb a mountain. In it the skirt is pulled wider than usual because she has to do some kind of semi-split to stay on the mountainside. I didn't pay a lot of attention, but it seemed to me that it looked like a half circle, as the dress looked like it took on a fourth circle shape naturally.
The scene is somewhere in the middle of the movie, but until the dvd is released I guess it's difficult to get any great detail out of it. Maybe the dvd will have some info on the extra material section though?
It does seem to have darts from this image
http://static3.wikia.nocookie.net/__cb20131007002322/disney/images/a/ac/Disney_anna_cutout_2013.jpg
The embroidery also seems familiar. I remember seeing pretty much exactly that pattern before, but can't remember where. Pretty sure it was in some kind of relation to these folk clothes though.
I hope that this helped a little and didn't confuse anyone.
Good luck with the projects, everyone who is working/planning them. While I'm generally iffy about the 3d movies by Disney, the clothing designs they do I find really nice.