Here's a quote from a book I have on Willow, written by the owner of Anglian Willow
'When we prepare clefts for making into mens bats, these are sawn to a width of five inches. The same is not the case for making 'boys' bats. These are sawn to a width of four and a half inches. When you consider after the finishing process a man's bat is four and one eighth of an inch wide, it does not take great guesswork to create, albeit with great care a man's bat from a boy's cleft. The thickness of a boy's cleft is less than that of a man's cleft and while it may produce a man's size bat, the batmaker will encounter problems carving a top class shape from the cleft'