Regarding question 2764, when istinja is obligatory?

Jazakallah for your advice, I though have trouble understanding your answer. You state that if the effected part (private parts) is more then a dinar (2.75cm), you must perform instinja with water. 1) Is this what you are saying? 2) If I urinate the efffected part is not bigger then a dinar, so i may use toilet paper, if a person does stool the effected part is more then a dinar, so it is obligatory to use water (this is every time a person does stool). 3) Is it obliagtory or sunna to use water for istinja or toilet paper is ok; Is the purpose only to clean? 4) If the effected part is more then a dinar and I use toilet paper does that make salah invalid? Please explain.