+ Positive Mitzvah
Separate dough from the batch and give it to the Kohen
“The first of your kneading you shall set aside Challah (a portion of dough)”
One of the five species of grain where the dough has the (minimum) size of 43 and a fifth eggs then separation is required.
Outside the land of Israel, the separation of the dough is according to the words of the Scribes to not forget the law of separation and this is burned.
From the Torah there is no set size and whatever dough you separate is sufficient.