The Gemara rules that to be obligated in Tzitzis, the garment must be large enough to cover the head and majority of the body of a child. The Gemara refers to a child who is old enough to go out into the market unaccompanied.
The Alter Rebbe in the Siddur rules that the Tallis Katan must measure at least 1 Amah (approx 49cm) across and 1 Amah in length at both the front and the back. The area of the neck whole is not included. Therefore, excluding the neck opening, the Tzitzis should measure 2 Amos total length. When buying a Tallis Katan one should give and extra allowance for shrinkage.
Once a child has reached the age of Chinuch (6 years old) they should be wearing a Tallis Katan that satisfies all of the Halachic parameters. A Tallis Katan that is 1 Amah long in both the front and back would be much too big for such a child. The Alter Rebbe writes that one can rely on the opinion that only requires the total garment to be 1 Amah in length by 1 Amah in width i.e. ½ an Amah in the front and ½ and Amah in the back.
From the age of Bar Mitzvah, even a small boy should only wear a Tallis Katan that is an Amah long in both the front and the back. Based on modern-day Tzitzis sizes, this is a minimum size 20 Tzitzis. It is preferable to use the next size up (22), to allow for shrinkage.
