BEST REAR FACING for long toddlers CAR SEAT 1 2 3 (isofix but not i size vehicle)

Hi all,

I have an almost 16 months old baby and I am in the process of choosing a car seat for him, I am looking for the safest rear facing (and also forward) car seat but I need to consider that my little one is a big boy and quite long and I want him to feel as much comfortable as possible. I have a Fiat that has isofix but it is not i size compatible. Could you please help me mums? Thanks in advance!