i make my bottom loop smaller- so it stays in the nock after un stringing, and the top loop big enough, as jasper said- so that it can slide down the limb- which is about 7cm "circumference" for my bows.
whilst building my bows i also open up the nock a bit on the sides and belly- on the tip side- so that at full draw, my loops is unaffected by the nock groove- easy for the nock to slide from a braced to a full draw- less wear and tear on the loop.
the only string i ever had break- happened there, where the nock groove wore through the string, in a few shots on a new bow- yup the edge was too sharp - an unfinished bow- but now that wont happen again.