Did Ibn Taymiyyah specify an amount for "slight impurities" that are pardoned, or is its estimation left to the individual? And what about slight irritation in the eye after prayer, and a small amount of something on the body after ghusl, especially when there is waswas (obsessive whispering) regarding this issue?

This issue pertains to pardoning a small amount of something that prevents water from reaching the skin, provided it is pure and not ritually impure (najis). The view of Sheikh Al-Islam Ibn Taymiyyah is that a small amount of dirt under the fingernail or in foot cracks, and any small impediment that prevents water from reaching [the skin], is pardoned. The criterion for this is common practice ('urf). It is preferable to remove anything that prevents water from reaching [the skin], even if it is a small amount, to avoid disagreement. However, if this is difficult, there is no harm in adopting the view of Sheikh Al-Islam, especially for those who suffer from obsessive doubts (waswās).