What size baby stroller blanket should I get?

The small blanket is roughly a square of 28" by 30". The large is a rectangle and is about 28" by 36."

Originally we thought the smalls were for infants and the larges were for toddlers. However, from our customers, we have learned that sometimes parents just want a buy a large so their baby can grow into it and so it is a large ground cloth for rolling around. So, really, personal preference.