Tzitzit come in two standard sizes, thin and thick. Either can be used on a tallit, though it seems thin is more commonly seen in modern congregations, while thick is the standard in traditional congregations.
If you are tying tzitzit yourself, thick may be a bit easier on your fingers.
On a tallit katan, thick tzitzit strings are rare.
The Shulchan Aruch says tzitzit should not be too thick or two thin, but medium. Apparently “medium” applies to both the thin tzitzit and thick tzitzit sold today, whereas in the past, tzitzit production was cruder in some ways and was on a very small scale, so the thickness varied considerably.
Occasionally you will come across medium thickness tzitzit (which we now sell). These can work on either a regular tallit or a tallit katan, and are a good option for those who simply can’t decide whether to go with thick or thin tzitzit.
To see our selection of tzitzit strings, click here>>