The smaller the leaf of the cactus, the smoother the pulp and the shorter the cooking time. If you're using cacti to roast or for a stew, the size of the leaf is not very important. Just make sure they're fresh, the skin is smooth, and they have a nice bright green color (not gray or dull). However, if you want to stuff them or blend them in a smoothie, it is better to choose the smaller, thinner ones as they are much softer and have no annoying fibers.