Haha, just suggested this on the Ohayocon forums. ^_^;; Guess I'll put it here too.
I once wrote up a brief tutorial for a shirt very similiar to theirs (Sai from Hikaru no Go) - the sleeves are smaller and it's shorter, but it's the same idea. I tried to adjust it for Haku's shirt instead of Sai's. Don't know if it will help, but I hope so.
I don't have the measurements I used as my sis has the costume, but I can give you an idea (remember to add room for hemming!):
BACK - The width should be about width of your chest area. The length should be from the top of your shoulders to the your thighs. The cut out for the neck should probably be about 7-8 inches, I'd say (adjust as necessary).
FRONT: OVER - The width is a bit trickier on this one. I added the mini model for this reason, so let me know if you still need an explanation. It should be at full width a little below the waist (as it poofs out a bit) to the bottom.
FRONT: UNDER - The width should be the back's width minus the number of inches of one of the shoulder "straps" (the pieces of material on either side of the cut out for the neck). The length should be from your shoulders to a little below your waist.
COLLAR - You see the red dotted line on the front and back pieces? Add up the length of that and that is the length of your collar. As for width, I'd say about 2 inches as you want enough to be able to fold it over on the dotted line and run a cord through it.
What are the dash marks on the back and sleeves for? That is how the sleeves are attached to the main portion. I'd say give about 5-6 inches for that and put them around your shoulder blades. And as for the clasp mechanism around the neck, eh, well, I'm not sure if I could explain it really well... I may have to tackle that later. Just know it consists of one knobby end (on the FRONT:OVER piece) and a cord that twists and hooks on to it (from the middle of the BACK).
There may be an actual pattern for this out there somewhere - I just winged it myself - and if so go with that (obviously), but if not, I found this works pretty well.