i-Size is the new EU safety legislation R129 that was ratified in July 2013 increasing the safety level of child car seats. The previous standard R44 04 will continue to run concurrently with the new R129 standard for several years to come. So if you have an existing car seat there is no need to replace it.
The main rule changes are :
Children must rear face for a minimum of 15 months.
Car seats will be classified and grouped by the height/length of the child rather than weight.
Side impact test introduced to R129
Improved side and front impact under higher forces.
i-Size favours isofix fitting as there is less chance of incorrect fitting.
i-Size seats will be compatible with all i-Size cars and most isofix cars.