How do you make a larger arc to make the loading and charging animation slower?

I think you are confusing some things. Right off the bat, I wouldn't create an item animation (if that is even possible). The best way of creating a "loading" animation would not animate the item, but animate the player to do the "loading" animation if that makes sense. Either way (even if you could make an item animation, but I believe it would be easier to just animate the player), player animations I am sure are not supported yet but may in the future. If you need an animation, I would look at Minecraft forums and follow a tutorial (there is plenty) and code it yourself.

Next, I am confused by, "How do you make a larger arc to make the loading...". <- This is vague, to make it go farther just give it more velocity? And create a modifer to give more velocity upon more charging. More charge = more velocity, add 0.5 velocity for every second of charging if that makes sense. Minecraft BowItem does this in this method if you need an example,

   public static float getArrowVelocity(int charge) {
      float f = (float)charge / 20.0F;
      f = (f * f + f * 2.0F) / 3.0F;
      if (f > 1.0F) {
         f = 1.0F;
      }

      return f;
   }