All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
The Hanafi School of jurisprudence stated that the size of the Dirham from a major impurity is forgiven and it is not obligatory to wash it. The prayer with the existence of such impurity is sufficient (i.e. valid).
The size of the Dirham here is specified in the books of the Hanafi School of jurisprudence.
Kanz ad-Daqaa-iq, which is a Hanafi book, reads:
“The size of a Dirham is pardoned like the width of the palm from a pure impurity, such as blood, wine, chicken feces, urine of animals that we are not allowed to eat, and dung; and what is less than a quarter of a garment from a minor impurity, such as the urine of animals that we are allowed to eat, the urine of a horse, and feces of a bird that we are not allowed to eat.” [End of quote]
Minhatul-Khaaliq, authored by Ibn ‘Aabideen, from the Hanafi School of jurisprudence, reads:
“His saying: what is meant by the width of a palm etc.), then Munla Miskeen said: “The method of knowing it is to take a handful of water and then you open your hand wide open; whatever is left is the size of the palm.” [End of quote]
As regards your saying "I know that we should wash any impurity even if it is a small amount if we are able….”; then this is not accurate and it contradicts the subject matter of your question, which is that the size of a Dirham is forgiven; so a person is not obliged to perform Ghusl even for a person who is able to do it.
For more benefit, many scholars are of the view that a small impurity is pardoned.
The Kuwaiti Fiqh Encyclopedia reads:
“The Maaliki School distinguished between blood – and the pus – and all other impurities, so they are of the view that the size of a Dirham of blood is pardoned, as well as the pus. What is meant by the Dirham is the Dirham of a mule, which is the black circle in the mule's front leg.
The Shaafi'i School of jurisprudence is of the view that little blood and pus are pardoned, as well as what is difficult to avoid and which is widespread, such as the blood of sores, abscesses, fleas, and what is hard to see with bare eyes, and bloodless creatures, and so forth. The criteria in what is little and what is much is the custom.” [End of quote]
In addition to the view of the Hanafi School that we have already mentioned.
Allah Knows best.