Siman 16
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א שִׁעוּר טַלִּית שֶׁחַיָּב בְּצִיצִית שֶׁיִּתְכַּסֶה בּוֹ בְּאֹרֶךְ וּבְרֹחַב רֹאשׁוֹ וְרֻבּוֹ שֶׁל קָטָן הַמִּתְהַלֵּךְ לְבַדּוֹ בַּשּׁוּק, וְאֵינוֹ צָרִיךְ אַחֵר לְשָׁמְרוֹ: הַגָּה: וְאָז חַיָּב בְּצִיצִית וְדַוְקָא שֶׁהַגָּדוֹל לוֹבְשׁוֹ פְּעָמִים עַרְאַי וְיוֹצֵא בּוֹ לַשּׁוּק (בֵּית יוֹסֵף בְּשֵׁם בֶּן חָבִיב וּבְשֵׁם מַהֲרִי"א):
1 The Measurement Of A טלית. Containing 1 Se'if The measurement of a טלית that is obligated in ציצית is [a garment] that covers, in length and width, the head and most of the body of a child who goes out by himself to the marketplace and doesn't need someone watching him. Rema: And then it is obligated in ציצית, and this [refers] specifically to when an adult would sometimes wear it in a temporary fashion to go out in it to the marketplace (Beit Yosef in the name of Ben Chaviv and in the name of Mahari"a).