In my experience, a 10" flour tortilla works the best. It is big enough to allow for a nice portion of filling and they are easy to wrap. Smaller flour tortillas will work, but you have to reduce your ingredients and they can be tricker to roll into the wrap. Larger flour tortillas will also work just fine, but you want to increase your ingredient amounts to make sure you have enough filling so the proportions are correct. Too little filling in a larger tortilla will result in too much of the tortilla on the ends and it doesn't make for a great wrap.