I'm using M&M's as my wedding favor. What size organza bag do I need and how many M&M's do I need?

I'm using M&M's for my wedding favors. I have invited 200 guests, but I'm thinking I'll only have 150 show up. I want to use organza bags, but I don't know what size to buy. Etsy has a lot of sizes like 3x4, 4x6, etc. Also, how many M&M's should I put in the organza bags? I was thinking about 15 or so. From what I've seen online, I would think that I would need to buy a 5lb bag of M&M's in my wedding colors. Any help you can give me is appreciated. Thank you!