Saw a short on making the movie, and it showed the actors practicing drawing and shooting that way. Looks like a mix of some Asian styles. Did not show if they were actually hitting well that way. Their bows were long and thin, and the arrows had waaaay smaller points than the CGI arrows in the movie.
As for the Navi...
If they are 9 - 10 ft tall, they could draw 48 inches?
If they weigh 600 - 800 lbs, their war bows could pull 200 - 400 pounds of draw weight?
Those arrows look like atl-atl shafts, they could weigh 3000 - 4000 grains?