The larger ones are best chopped and cooked long and slow. I've tried cooking them quickly, like spinach, and have not been happy with the leathery results. For a liter of tea, use 3-5 medium size leaves with whatever blend you favor. Soak the leaves in water with a disinfectant such as Microdyn, before using, as you do fruits and vegetables. Use pottery or glass.
Percentages are of minimum daily requirements.
Here's what we've found out. It's common practice in Mexico to use the leaves raw in agua fresca, a tea-like cold drink, and our source brochure from the Mexican federal agency for family development doesn't indicate there's any danger. Those who live in areas with Spanish-speaking populations might try grocery stores or nurseries in Latino neighborhoods. If you get a positive answer, ask them to write it down.