In addition to the posts above, I'd say the best starting place for width is the boards reference stance width. Unless you're abnormally tall or short for your weight it'll be a good all around starting place. Should be somewhere around 20-21" or so. Going wider isn't always better...
As for stance angle, some guys swear by forward angles, some guys swear by symetrical duck (both bindings toed out the same angle usually -9/+9 to -15/+15 or so). I have tried them all over the years and prefer -9/+9 for my freeriding. I find it easier to ride switch and it doesn't hurt my carving at all.
Best bet is to setup the bindings then stand on your board and try bending your knees/doing a squat. If one of your knees hurts change the angles. I get the most mobility for MY bones with my setup, you might not be the same.