I don't remember where I read this but I do use it at a guide: the diagonal measurement of a rectangular board should be 4" longer than the knife you use. My 10" knife has an overall length of 14.5". My go-to cutting board is 12" x 18" yielding a 21" diagonal. When I use my smaller cutting boards I do notice the difference. I would imagine for a round board you would want the board's diameter to be knife length + 4".