"i-Size" does, in fact, refer to the size. And indeed height is the basic criterion for determining when to change a child seat for a bigger one. With the new i-Size standard, the age and weight categories have been left behind. Height is now the determining factor and overrules the previous division into groups.
Child restraint systems with i-Size certification are no longer divided into categories or groups by weight. The manufacturer is responsible for deciding for which children the child seat is safe and effective depending on the product's features. You will now find that many i-Size-certified child seats state whether the CRS is suitable for a child of a certain height.
Previously, with the former categorization system, many parents used to change their child seat to a larger one earlier on, forcing the child to face the direction of driving when it has been shown that the safest way for children to travel is facing backwards for as long as possible. Indeed, Fundación MAPFRE recommends that children do so up to the age of four. The i-Size certification allows children to travel facing backwards until the age of 15 months.
Experts believe that height is the most reliable factor when choosing a child seat. Among other reasons, it has been found that parents are more aware of their child's height than, for example, their weight. This falls in line with the system for choosing children's clothing.
There is no point taking age as an indicator as there can be huge differences between children of the same age. Every child grows at a different rate. The height difference between children of the same age is every more marked in children of different nationalities. We should not forget that one of the objectives of this law is to unify child restraint systems both Europe-wide and globally, which makes height a good indicator.
In spite of this, age and weight may still be used as indicators to help parents choose the best CRS.
In July 2013, the first phase of the new i-Size standard went into effect. At the moment, both systems are accepted: both the ECE R44 and the i-Size. This will remain the case until 2018, when it is expected that the new standard will definitively replace the former one.
In short, if you want to buy an i-Size-certified child seat, you won't have to consider what group your child falls into. All you have to do is choose a child seat in the height range for your child.