In my chain maille designs sometimes I want a certain size loop so it fits with the other jump rings in the design. The easiest way to do this is to use a jump ring or a loop that you already have that is the size you want. This loop can then be placed on your round nose pliers and you can mark the spot that you need. Marking the spot with a sharpie or other permanent marker saves time and ensures that your loops are uniform. You can mark the metal jaws or like this set of round nose pliers you can mark the nylon. These marks can be removed fairly easily by simply rubbing lightly with a paper towel or fine sandpaper or wiping with rubbing alcohol.

If the loop you want to replicate is already attached to a design and won't go on your pliers you can use verniers or calipers to measure the inside diameter of the ring and transfer that to the round nose pliers.