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.
Paraphilic infantilism is the psychoanalytical term used for those who like to engage in being treated as an infant or toddler. Adults who engage in play mimicking an infant are known as adult babies. About one in three adult babies are also referred to as diaper lovers. The majority of adult babies are heterosexual males. Adult babies, whether diaper wearers or not, historically do not have a sexual preference for children. Adult baby fantasies include being free of guilt, responsibility, or control and while some act completely like babies at times, others practice in discretion.