What is i-Size?
“i-Size” is the name of a new European safety regulation that affects car seats for children under 15 months of age. It came into effect  on July 9th 2013 and provides extra protection in several ways, most notably by providing rearward facing travel for children up to 15 months in an i-Size car seat instead of between 9 to 12 months, which the current EU regulation advises

No obligation to upgrade
The i-Size regulation does not replace the current ECE R44/04 regulation, so you do not need to upgrade your existing child car seat. If you currently use a rearward facing child car seat that is fixed to the car by the seat belt, you can continue to use it. The introduction of i-Size simply means that you now have an extra option when buying a car seat for your baby/toddler – an option that complies to the new i-Size safety regulation.
But it is safer
Child car seats that comply with the new i-Size regulation provide up to twice as much protection as conventional child car seats. For example, i-Size seats provide better head and neck protection, thanks to longer rearward facing travel up to  15 months. i-Size also provides better side-impact protection because, for the first time ever in a child car seat regulation, it includes a set of minimum performance criteria for this type of collision.
i-Size seats are also much less likely to be installed incorrectly, because they have a simple click-and-go installation system  that does not involve the car’s seat belt. In addition, the i-Size regulation will help prevent moving your child to the next stage car seat too early – specifically, from a rearward facing seat to a larger forward facing one. That’s because it has a clear specified requirement of 15 months old and/or 75/80 cm in length, instead of 9 to 12 months. The chance of moving your child to the next stage car seat too soon is also minimised because the child’s length is used in the deciding factor and parents will not mistakenly conclude that their child has outgrown the first seat his feet are outside the seat.
And finally, i-Size means more peace of mind. Not all current child car seats fit all car models securely. However, all i-Size car seats do fit most cars equipped with the ISOFIX system and all cars with i-Size seating positions.
The 5 Key Points of i-Size
1. Improved protection at higher forces from side & front impact and a much better protection of head and neck
2. Rearward faced travelling mandatory up to 15 months old
3. i-Size also requires Isofix, which has less chance of being incorrectly used than belted car seats
4. i-Size car seats will fit all Isofix cars
5. Length classification fro easier choosing the right car seat, similar to clothing sizes