Praise be to Allah.
If a man has intercourse with his wife during the day in Ramadaan, each of them must offer expiation, which is freeing a Muslim slave. If they are unable to do that, then they must each fast for two consecutive months if she was willing. If they are unable to do that then they must feed sixty poor persons, each of them giving thirty saa’s of the local staple food, one saa’ for each poor person, half on behalf of the man and half on behalf of the woman, if they are unable to free a slave or fast. They also have to make up the fast for the day on which intercourse took place, and repent to Allaah and turn to Him, and regret it, and give it up, and ask for forgiveness, because intercourse during the day in Ramadan is a great evil and it is not permissible for anyone who is obliged to fast. End quote.
Majmoo’ Fataawa al-Shaykh Ibn Baaz (15/302).
Based on that, the amount of food that should be given to the poor person is half a saa’ of rice etc., i.e., approximately one and half kilograms.