The smallest width of an infant car seat is one with the smallest base of around 15.5 inches, the Combi Coccoro. It is a lightweight infant car seat and is the narrowest car seat amongst the rest as yet. The best part about this narrowest infant car seat is that it has a rear-facing view that grows with your child. However, the only downside is that the car seat by combi coccoro only expands up to 25 inches in width, not more.