Not sure that there is a best size but I do know that most garages on builder's plans, in spec houses, and on the plans available online are way tooooo small. Typically I think they're often just 20x20.
Merriam-Webster dictionary simply says a newborn is a baby who is recently born and does not put an upper limit to the term. The dictionary also defines an infant as a child in the first stage of life but doesn't give any age specifics. The World Health Organization (WHO) defines a newborn, or neonate, as a baby under 28 days old. As the name implies, a toddler is classically defined as a child who is just learning to walk or one who toddles. This is often around one year of age. There's no official definition of the upper limit of toddlerhood.
1. preemie-an infant that is born prior to 37 weeks of gestation. premature baby, premature infant, premie, preterm baby, preterm infant. neonate, newborn, newborn baby, newborn infant-a baby from birth to four weeks.