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I made sure mine were long enough when I was in a sitting position, to keep wind from going up my leg. It makes them baggy when I stand up but that's okay with me.
Scissors work fine. I would leave them a couple inches longer than my pant leg so they don't ride up.Trim about half of what you think you need and take a ride. It is a lot easier to trim more off than to add some back on if they're too short. JMO
Sit on your bike like you were riding and have someone measure to ankle and mark chaps.Then stand up straight and make sure they are not going to drag on the ground.A good pair of scissors should do the job in the right hands.