Can one make a bracha on tallis kattan, if the strings are shorter than double the knots? Is this just a hidur. Explain further. What if its longer, should one cut the strings ?
One can make a bracha on such a tallis katan. The length of double the knots is in fact just a hiddur. One may cut the strings in order to adjust the length, however not with a metal knife or scissors.
The gemara in Menachos 39. brings that it is a beauty for tzitzis when the strings are double the knots, this is codified in O:C 11:14. The Mishnah Brura ibid. points out that this is not a requirement which is essential, one fulfills the mitzvah even without this ratio.
The Rama, Siman 11:4 writes that one may cut the strings in order to adjust their length, and there is no problem of תעשה ולא מן העשוי.
The Magen Avraham ibid. s”k 18 brings from earlier sources not to cut tzitzis strings with a metal knife. Early sources recommend cutting with ones teeth. Today this is generally difficult and destructive to the strings. A ceramic knife or scissors is handy for this purpose, or even a shard of glass.
The halachic requirement for the length of the strings is that they be from the beginning of the knots until the end of the strings the length of 12 “agudlim”, see Siman 11:4. Practically this is between 24 and 30 centimeters. If the tzitzis were made the proper length and afterwards torn there is more room for leniency. The complex details of this halacha are delineated in Siman 12.