Ensuring that your tallit katan meets all the requirements to fulfill the mitzvah of tzitzit is not as simple as you might think. Although generally speaking, there's sort of a general understanding that if you have a standard cotton tallis katan in Size 8 (US Size 20) or bigger, or a standard wool tallis katan in Size 5 (which is the same US Size 20), then you're okay.
But if you go into the halacha, you'll find that there are numerous details and stringencies worth addressing. In fact, the Mishnah Berurah cites a common custom of not making a blessing on a tallis katan at all, but rather relying on the blessing you recite on the tallis gadol.
Just familiarizing yourself with the issues takes some time and effort. Cotton or wool? How wide does it have to be where it rests on the shoulders? How wide can the neck opening be? How far must it come down in front? If there's a slit in front, how do we take measurements? Does it have to be an amah wide, or is three-fourths of an amah okay? The list of questions goes on and on. And we haven't even gotten to the strings yet!
To fulfill all of the requirements, often you will opt to go either according to the Mishnah Berurah or the Chazon Ish. (I suppose there might be a "Rav Moshe Feinstein tallis katan" as well, but here in Eretz Yisrael generally the discussion will be according to the Mishnah Berurah approach or the Chazon Ish.)
Today I came across an outline of the Chazon Ish's approach in Shoneh Halachos, which was written by HaRav Chaim Kanievsky decades ago, and which incorporates many of the Chazon Ish's rulings. The book seems to be unchanged for many years, so if you want to look this up, you're likely to find it on page מ"ו, right at the end of the halachos of tzitzis, which you can access online here on Hebrew Books.
In Shoneh Halachos it's formatted in one very long paragraph, but I'm going to do my best to translate it formatted as separate points, leaving out all of the citations.
- The garment should be made of wool or other fabrics, but not linen.
- It should be white.
- The edges should be reinforced.
- Including the reinforcement, the measurements should be 120 cm by 60 cm.
- Those who elect to be stringent, to fulfill the Mishnah Berurah, should make it 60 cm by 60 cm beyond the bottom of the neck opening.
- In both cases, it is advisable to make it slightly larger to allow for shrinkage in the laundry.
- It should not have sleeves, there should be a neck opening in the center and one should ensure that each shoulder piece is wider than the neck opening, and the length in front and back should also exceed the neck opening.
- A square fabric reinforcement measuring three fingerbreadths by three fingerbreadths should be sewn on each corner.
- Each corner should be at a right angle, without loose threads.
- The holes should be positioned 4.5 cm from the edges, and should be reinforced, but not with threads made of the same material as the garment and not wool threads (and according to the Mishnah Berurah, ideally they should be colored threads).
- Take four wool strings which have been carded and spun and twined with mitzvah intent, by an adult.
- They should not be excessively thick or thin, they should be white and ideally they should be folded into eight.
- The strings should measure at least 30 cm beyond what is tied on the garment, and preferably longer.
- One string, which is wrapped around the others, should be longer.
- The strings should be paid in full before they are inserted into the garment.
- Before insertion, say aloud, 'The tying and all that I do I am doing in order to fulfill the mitzvah of tzitzis.'
- Insert, fold in half and even out the two sides.
- Take four strings on one side and four on the other and tie a double knot.
- Wind the long string around seven times, tie a double knot, wind around eight times, tie a double knot, wind around eleven times, time a double knot, wind around thirteen times and tie a double knot.
- Make each section of equal length, with one thumb breadth from one knot to the next. Altogether the tied part should be at least 10 cm.
- If the tied section is longer than that, make sure the loose section is at least twice the length. And the loose section can be longer than that as well.
- The tzitzit should hang down alongside the tip of the garment.
- If the tips start to fray, it is advisable to make a knot at the end.
- Ensure that the strings are not tangled.
- Do not tuck the tzitzit in your pants, but rather leave them showing.
- One who buys a tallis katan and ties on tzitzis should recite Shehechiyanu once the tzitzis are tied on.
- If he possessed the garment previously, he should recite Shehechiyahu on a piece of fruit and include it in the blessing.
- It is a mitzvah to make an attractive tallis with attractive tzitzis.