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.
A small child is commonly referred to as a baby irrespective of his age though there are developmental stages in the first few years of the life of a human offspring. The smallest of babies are the newborns in the first few days of their lives. Infant is a word that is reserved for a kid before the age of 1 year. So, if you have a baby boy or girl who is a few months old, the doctor would refer to him or her as an infant. However, there are many grandmothers and grandfather who would take exception of this definition and call their grandchildren babies even when they are much older because of their extreme love for their grandkids.