The American Camelbak makes several products where you need a size chart. Almost all of their hydration packs come in "one size" and are very adjustable. For some other products it is best to measure them first.
Camelbak Size Chart
Below is the size chart for the Camelbak Ultra Belt.
• Size XS/S: waist circumference between 66 and 76 cm
• Size S/M: waist circumference between 71 and 81 cm
• Size M/L: waist circumference between 76 and 89 cm
