Obviously if you were to put tzitzis on a garment that measures just a few inches, nobody would claim you can fulfill the mitzvah with a garment that small. So what is the minimum size requirement for a tallis katan? The answer is actually quite involved.
The Gemara and Shulchan Aruch tell us that a tallis katan has to be big enough to cover the head and majority of the torso of a boy who is old enough to walk around a central public area alone.
שעור טלית שחיב בציצית שיתכסה בה בארך וברחב ראשו ורבו של קטן המתהלך לבדו בשוק ואינו צריך אחר לשמרו
According to the poskim, this refers to a boy around the age of nine. In other words, if the garment is big enough for a nine-year-old boy, it’s big enough. Other poskim debate whether it also has to be big enough so that an adult would not feel embarrassed wearing it in public.
However, since clothing styles have changed so dramatically, it’s not easy to translate this into practical size guidelines. Some poskim hold that the total length of the tallis katan (front and back) must be one-and-a-half amah, while others say it must be two amos. The prevailing opinion is that the width must be one amah.
How big is an amah? That’s also a matter of debate. There are three main opinions:
Grach Na’eh – 48 cm (18.9 inches)
Rabbi Moshe Feinstein – 54 cm (21.2 inches)
Chazon Ish – 58 cm (22.8 inches) or 60 cm (23.6 inches)
Today most people follow Grach Naeh (especially Sephardim), which is size 20, Rav Moshe (especially Americans), which is size 22, or Chazon Ish (which is size 24), especially people in Bnei Brak and many kollel-leit and bnei Torah elsewhere.
But before you take out your tape measure, first we need to know how to go about measuring the tallis katan. Does the neck opening count or not? If there’s a slit in front, does that count?
According to the Mishnah Berurah, the neck opening should not be counted in calculating the dimensions, i.e. you would measure from the bottom of the neck opening down to the hem. However, the Chazon Ish writes that if the material on each shoulder is wider than the neck opening, then you can measure from the shoulder down to the hem.
This creates an interesting situation: The Chazon Ish has a more stringent amah, but the Mishnah Berurah holds by a more stringent way of measuring the tallis katan, therefore the difference between the minimum requirement according to the Mishnah Berurah and according to the Chazon Ish, respectively, is not so significant.
Mishnah Berurah 4, ibid.
O.C. 10, 7