For width, you want the nock to fit without being pinched, no more. If you do more, you can get inconsistent nock points and the arrow might move during draw/shooting depending on the tightness of nock fit.

For length, longer is better for forgiveness, one of the things a loop does is reduce twist on the string as you draw, longer is better for this. However longer means you need to either move your anchor point (bad idea) or shorten the draw length of the bow.

--Bob