In the recipe for shiso(periila)pesto you say 25-30 leaves but they are different sizes, so my question is about how much cups worth?

I use a different recipe to make mine, but it calls for 1 ounce of leaves, or about 25 large leaves. I tend to not worry so much about the exact, precise amount to be honest with you. Generally I use between 20-30 leaves of various sizes- because that is how they grow after all. I try to get at least the 1 ounce, but if it's a tad less or a bit more it all works out just fine. If you don't hear from the recipe author, and as it's been more than 12 years since it was posted- you may not, then try my suggested amounts.