"Preemie size" refers to clothing or items that are specifically made for premature babies or infants who are born before their expected due date. Full definition
Rumparooz Lil Joeys: The Lil Joeys are by far the smallest fitting newborn diaper we've found without going into preemie sized diapers. (theclothdiaperwhisperer.com)
Baby clothing begins with preemie sizes, which will usually fit the tiniest babies, those under 6 pounds. (thebabycorner.com)
The hats are even available in preemie size, to accommodate premature babies. (checkpregnancy.com)