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.
term infant. Etymology: L, terminus, limit. any neonate, regardless of birth weight, born after the end of the thirty-seventh and before the beginning of the forty-third week of gestation. Infants delivered at term usually measure from 48 to 53 cm from head to heel and weigh between 2700 and 4000 g.