Take a tape measurer (or string if you don't have one), and measure the circumference of your chest from right under the armpits going all the way around.
Extra Small: 70-80cm (chest size), 30-50kg (weight)
Small: 81-90cm (chest size), 51-70kg (weight)
Medium: 91-105cm (chest size), 71-90kg (weight)
Large: 106-120cm (chest size), 91-120kg (weight)