Note: Updated Jan 2011 with the new larger BSA neckerchief size. Updated again Sep 2018 with new URLs and the 2015 edition of the ANSI standard.
The BSA’s Insignia Guide says this about the size and shape of the neckerchief, “Official neckerchiefs are triangular in shape.” There is a more info about how to wear it, who chooses the neckerchief (the troop), who approves special neckerchiefs (the council), and so on. It does say that special neckerchiefs are “the same size as official ones”. Oddly, they don’t say what size that is.
So I researched it. One reason to have a bigger neckerchief is so it can be used as a triangular bandage, so I also checked the common sizes for those, including the ANSI-standard size.