The scale below gives an indication of suggested size for shin pads:
X-Small = up to height 115cm / 3ft 9 inches
Small = up to height 135cm / 4ft 5 inches
Medium = up to height 155cm / 5ft 1 inch
Large = Over 175cm / 5ft 9 inches
X-Large = Over 195cm / 6ft 5"
To ensure the correct fit try the shinpads on before commencing play