Is there a minimum size that a Tallis Kattan has to be in order to be Yotzei the Mitzva of Tzitzis according to all Poskim? If there is a minimum size, what is it?
The main shiur is that which will cover the head and most of the body of a child who can walk independently in the marketplace (menachos 41). The Tur gives an age of 9.
There is a dispute about the second condition of the braisa that an adult would wear it in the street occasionaly. The Tur holds this is a second precondition and so the Rema adds this even though the mechaber doesn't ((O.C. 16:1- see magen Avraham). The 2nd precondition alone would not be enough of a shiur. However, the Mishna Berura (16:2) says that it seems from the Bach that if an adult (13 y/o) would wear it on a regular basis, you wouldn't need the precondition of the child.
BOTTOM LINE: The Mishna Brura (ibid.) quotes a shiur of 3/4 Amah L x 1/2 Amah W, but then quotes the Artzos haChayim who say this shiur has no basis and says that "Anshei maase" are particular to have an Amah on each side, but you should have at least 3/4 of an Amah, since the "Mesamchei Lev" measured a 9 y/o and found him to be 1.5 Amos.
Twenty-four inches wide. Some poskim hold that it is eighteen inches.
haRav David Yosef rules in Halacha Berura 16:1 (presumably based on a ruling of his father Hacham Ovadia) that unless a talit is 2 amot long (96 cm) and 1 amah wide (48 cm), not including the hole, one should not make a beracha.
Basing himself on Ritva, Nemukey Yosef, and popular practice, the Aruch Hashulchan (OCH 16:5) writes that the minimum sizes are only required for ittuf; wrapping oneself. Since we don't do this with a normal tallit k'tanah, there is no minimum shiur:
ולענ"ד כל חקירה זו בחנם והעולם מקיימים מצות ציצת בטליתות הקטנים דכבר בארנו בר"ס ח' מהנמק"י ומהריטב"א ... ומן התורה כל בגד של ד' כנפות שנושא על גופו חייב בציצת ע"ש וגם דעת רבינו הב"י כן הוא כמ"ש שם והא דבעינן עיטוף רוב הגוף זהו בטלית העליון ... אבל טלית קטן שתחת הבגדים א"צ שיעור כלל דאם רק יש בו ד' כנפות חייב בציצת וכן מבואר להדיא מדברי הריטב"א והנמק"י שהבאנו בשם בסעיף ד' ע"ש
"In my humble opinion, this whole investigation [regarding the requisite size of a garment] is pointless, and the public fulfill their obligation to wear tzitzit with small garments, for we have already explained in the begining of ch. 6 on the basis of Ritva and Nemukey Yosef...every garment with four corners needs tzitzit, and this is the opinion of the Beit Yosef. And the requirement to fully wrap oneself with a tallit, is only with a tallit g'dolah." (Trans. my own).
Big enough that if you wore this over your torso in the street, you wouldn't be embarrassed.
(Now Groucho Marx would ask what your torso was doing in the street, but that's a different question ...)
To make it simple. It would be size 22 of Tzitzis.