Preheat the broiler. In a pot of boiling, salted water, cook the jumbo shells until al dente. In a smaller saucepan of boiling, salted water, cook the baby shells until al dente. Drain separately, reserving 3/4 cup of the cooking water. Spread the jumbo shells out on a work surface.
Meanwhile, in a large, ovenproof skillet, toss the tomatoes with 3 tbsp. EVOO. Broil for 15 minutes. Remove, then stir in the garlic, herbs and 1/2 cup reserved pasta water; season with salt and pepper. Lower the oven to 350 degrees .
In a bowl, toss the baby shells with the remaining 1 tbsp. EVOO. Stir in the spinach, ricotta, 1/4 cup parmesan and remaining 1/4 cup pasta water; season. Stuff the jumbo shells with the spinach mixture; arrange in the skillet atop the sauce. Top with the remaining parmesan, cover and bake until heated through, about 30 minutes.