The American company Camelbak makes various products for which you need a size chart. Almost all of their hydration backpacks come in "one size" and are very adjustable. Some other products you might want to measure 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
