Is there a minimal size of a garment with four corners that makes it duty to tsitsit? Is there a one size fits all?
The size of clothes that need Tsitsit is treated in Gemara Menachot 40b-41a:
מאי טלית פטורה? אילימא דלית בה שיעורא, והתניא: טלית שהקטן מתכסה בו ראשו ורובו והגדול יוצא בה דרך עראי - חייבת בציצית, אין הקטן מתכסה בו ראשו ורובו אע"פ שהגדול יוצא בה עראי - פטורה,
What is meant by 'a garment exempt from zizith'? Does it mean a garment smaller than the prescribed measure? But it has been taught: A garment with which a child can cover his head and most of his body, and in which a grown-up person would walk out for a moment, is subject to zizith; but if a child cannot cover with it his head and most of his body, even though a grown-up person might walk out in it for a moment, it is exempt.
"A child" is not an accurate measure! To determine with which a grown-up preson would walk out for a moment is not easy. How this Gemara leads to precise sizes ?
The first step toward practical indications is to learn Rambam, Ahava, Tsitsit, 3, 1:
א כְּסוּת שֶׁחַיָּב אָדָם לַעֲשׂוֹת בָּהּ צִיצִית מִן הַתּוֹרָה--הִיא כְּסוּת שֶׁיֵּשׁ לָהּ אַרְבַּע כְּנָפַיִם, אוֹ יָתֵר עַל אַרְבַּע; וְתִהְיֶה מִדָּתָהּ, כְּדֵי שֶׁיִּתְכַּסֶּה בָּהּ רֹאשׁוֹ וְרֻבּוֹ שֶׁלַּקָּטָן הַמְּהַלֵּךְ לְבַדּוֹ בַּשּׁוּק, וְאֵינוּ צָרִיךְ אַחֵר לְשָׁמְרוֹ וְלֵילֵךְ עִמּוֹ;
A garment to which the Torah obligates a person to attach tzitzit [must meet the following requirements]: a) it must have four - or more than four - corners; b) it must be large enough to cover both the head and the majority of the body of a child who is able to walk on his own in the marketplace without having someone else accompany him and watch him;
The question is how Rambam assumed that the age, the development step of this child is determined by his hability to walk on his own in the marketplace without having someone else accompany him and watch him? The Raavad answer that this is a deduction, such a garment must be possible to help a grown-up person to walk for a moment (a passage of the Gemara not pasted on Rambam), so this kid must be close to the size of an adult person (an adult is at least 13 years old according to Halacha). The Kessef Mishne infered from the fact that the passage concerning adult person is skipped by Rambam, that this is an other Halacha: An adult, if large sized, cannot simply keep a Talit which can cover head and most body of a child. It must suit him a minimum, per example he will not to be ashamed with this garment and be able to walk. So i Reuven is XXL and Shim'on Small size, the Talit of Shim'on is perhaps unable for Reuven's Mitsvat Tsitsit.
The Tur define the child as 9 years old. Kesef Mishne explains that this age is linked to the ability to be careful not to loose the Talit, not only the ability to walking alone on the streets. Magen Avraham understand that 9 years is choose because this is the age of Chinuch.
Shulchan Aruch Orach Chayim 16
He specifies that the size regards length and width (head and majority of the body in length and width), that the child must to fulfill the condition mentioned above for understanding the Tur. So we may deduct 9 years.
So, it is very relevant to verify if it is not too small for walking a few moment with it on ths street.