Measure between the shoulders and the arms to figure out where you need to add how much.
Knitted cuff to cuff makes it easier. You just knit rows until it measures the right length then go on to the next step.
Keep track of what you did on one arm so you can do the same on the other.
Between the shoulders you may have to knit half the length you need so you can add the extra rows in the center. (I don't have any idea of what goes on between the shoulders on a shrug.)
If it's something like cables you have to figure out how to stuff in the extra here and there so it balances out.
12" or 15" armspan? Did you mean arm length?