We use a standard American sizing chart by showing inches and pounds.
Industry standard for baby apparel is to use age as the size, but it is important to measure your child as it is possible to have an older child that still needs a smaller size, or conversely a younger child that needs to size up. If when measuring your child falls into two different sizes between weight and length (i.e. 27 lb and 36"), then it is best to size up to ensure a better fit.
How to measure:
Height - measure height from the top of the head to the bottom of the foot.
Weight - note child's weight to the nearest pound.
Chest - with arms relaxed at child's sides, measure around the fullest part of the chest.
Waist - aligning the tape measure with the child's belly button, measure around the narrowest part of the waist.
Inseam - measure from the top of the inner thigh down to the floor.