What is the ruling on water mixed with urine splashing from the toilet onto the body when defecating, and is it considered a small amount of impurity that is pardoned if it is less than the palm of the hand, and if its amount is unknown, is it pardoned?

The criterion for a negligible amount of impurity that is pardoned is the size of a Dirham. This is the view of the Hanafis. When in doubt, the default is that it has not reached the limit of a large amount, and the capacity of the palm's interior is not taken into consideration. It is more prudent to wash whatever impurity affects one's clothes or body until one is reasonably certain that it has been removed.