Cut the sleeve edge at least 22 inches wide - these are jedi robes and should have super wide sleeves. Wider than you would ever think you could possibly want. You can always cut them smaller if you desire, but trust me, giant sleeves look awesome.
The open edge of your cut out sleeve should extend 8 inches beyond extended fingertips - this makes for bigger sleeves and they look good bunched up on the arm a little bit, rather than smooth like wizard robes. Remember, you are NOT cutting the top edge.
The very top armpit edge of the robe will be the most narrow point - flare the cut away from the point and out, curving inward again at the bottom edge to make everything circular at the bottom. Cut less than you think you need, you can always sheer it off later.