i-Size, or Regulation R129, is a car seat legislation that uses a new way of crash-testing car seats for babies and children, including a new side-impact test. i-Size seats are chosen based on the height of a child to ensure a child is the correct size for the seat.

i-Size car seats are considered to be the safest, with improved head and neck protection. That's because: 

Side-impact crash test results are assessed as part of the i-Size standard. 
i-Size safety tests are completed using advanced Q-dummies, which have more sensors than the dummies used for the R44 test.  
Your baby must travel in the very safe rear-facing position up to the minimum recommended age of 15 months. Some i-Size car seats can even be used rear-facing until your child is 105 cm tall, which is around four years old. Research found that rear-facing car seats can offer up to 75% more protection if your young child is involved in a car accident.
All i-Size seats promote and are fully compatible with ISOFIX cars, so if your car has isofix, the i-size will most probably fit.

• Car seat is suitable for newborn babies and children with height from 40 up to 150 cm;
• Car seat can be used "REARWARD FACING" for children with height 40-105 cm, age 0-4 years, approx. weight 0-18 kg;
• ECE R129/03 (UN/ECE) approved and certified.