Philips health band comes in two sizes:

S/M: for wrists of circumference  130-180mm / 5-7 inch and L/XL:  for wrists of circumference of 180-225 mm / 7-8,8 inch.

It is best to measure your wrist circumference at the location where you will wear the health band. You can also use the sizing guide printed on the box to determine the right size for you.