Carry all the important things and nothing else! 
Small size is just larger than a credit card and is great for cards, cash, 2 tubes of lip gloss, or 2 essential oil rollers. 
Medium is double the size of Small and can also hold the things mentioned above as well as small cell phones. 
Large will hold larger cell phones and everything mentioned above. 
Belt-Loop Pouches are the same size as a Small pouch, except it has no neck strap and instead has a loop and button on the back for attaching it to your belt loop. 
Twist Headbands fit Children roughly age 10+ as well as adult Women. My daughter and I can both wear them comfortably.