Impurities

Repeating salaah if one finds impurity on one's clothing

Q:

1. If one finds any impurity, like a small amount of stool on his clothes, would the clothes be considered napak and would the prayers offered with such clothes need to be repeated?

2. What amount of a najasat renders a cloth napak and impermissible to pray with?

3. If I need to repeat the prayer, would only the Fardh and witr be repeated or also the sunnat and nafl?