The Compendium of Caraka 1.5.4

1.5.4

yāvaddhyasyāśanamaśitamanupahatya prakṛtiṃ yathākālaṃ jarāṃ gacchati tāvadasya mātrāpramāṇaṃ veditavyaṃ bhavati//

For as much food eaten as undergoes digestion in due time without harming one's constitution, that much is to be known as the standard measure for that person.