There's no set answer for everyone. There's nothing magical about a TomCat that it requires a particular size. It all depends on how long the A2A of the bow is, the draw length, how far the sight is extended from the bow and the diameter of the pin guard, and how far the peep is from your eye. As mentioned the most common method used today for picking the right size is to encirclethe pin guard within the peep. To do this properly you need to go to a shop and try several differnt sizes.