Some Achronim write that the length of a garment that would cover a nine-year old measures at least an amah and a half. Some say that it is at least two amos. The poskim disagree about whether the width of the shoulder areas that surround the neck hole is counted toward the minimal dimensions. It is proper for the garment to be at least an amah wide. Chazon Ish explains that if the width of each of the shoulders is wider than the hole, the hole can count towards the width of the garment. In his view, an amah is approximately twenty-four inches. Therefore, by way of example, if each of the shoulders is approximately nine inches wide, a six-inch neck hole would bring the total to the necessary width.
[משנ"ב טז, ד; ביאורים ומוספים דרשו, 2 ,5 ו־6 ,וראה שם, 7 ,ו־יח, 6]