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.
Infant - various definitions. Toddler - older. Explanation: In medicine, an infant is a child under 12 months (at least in the UK). However, in UK schools, the 'infant years' are (or used to be - the system is different now from when I was at school!) the period from ages 4 to 7. The child would then progress to junior school (often within the same school) at age 7. 'Toddler', as far as I know, has no definition in medicine and is colloquially used of a child who is big enough to walk but is still considered a baby - 'toddling' is the kind of unsteady walking associated with young children.