What is the proper shiur (in either inches or cm’s) for a tallis katan for an adult (Length and Width of the beged)?
There are different levels, the preferred level is that it should be an amah in front and an amah in back, there are those who say that 3/4 of an amah is also alright. There are also differant opinions as to how big an amah is. The best amount which would fulfill all opinions would be 60 cm. by 60 cm, almost 24x 24 inches. (It is really 58cm but the tzitzis will shrink). According to R’ Moshe Feinstein Zt”l an amah is 21 1/4 inches 55.25 cm, and those that want to rely on the opinion of R’ Chaim Noeh it can be 49 cm. x 49 cm. Again it is worthwhile to add a little bit for shrinkage.
One can also be meikel and have it 44 cm. which is the amount of 3/4 of an amah.
Piskei Teshuvos 16-2, Igros Moshe Y:D 3 -64 (1).