How much is a tefach in width and length in regards of showing a tefach of hair when covering you hair?

Shalom, Thank you for your question. There are many different opinions about how much hair a married woman may show outside her hair covering. However, you are correct in thinking that many Rabbis allow showing up to a tefach of hair. How is that to be measured? Rav Moshe Feinstein zt”l understood this to mean the size of a square tefach – that is 8cm by 8cm, or according to some opinions up to 10.4cm by 10.4 cm. This square measure can be shown in any way – so, for example, a woman may leave out ½ a tefach by 2 tefach (this is approximately the width of the face – so if half a tefach was left out showing for the entire length of the fringe above the forehead, altogether that would not exceed a square tefach). However, some Rabbis explain that as long one of the dimensions is less than a tefach (8-10.4cm) then it is permitted, even if the length would be greater than a tefach. Based on this, one may leave the entire fringe along the forehead showing, even if it hangs down to (just less than) a tefach. This is based on the understanding that when the Talmud said a “tefach” it did not mean a square unit, but rather a strip that is not wider than a tefach. (Rav Y.H. Henkin zt”l). There is also room to leave some hair showing at the back of the head – as the hair at the front and back are never seen at the same time, so each would have it’s own “tefach”. I hope this is of some help, Blessings.