Yosef Karo

“ Paragraph 4 - If there are two individuals such that part of one's amot fall in the other's amot, like if there are six amot in between them, they may bring food to the center and eat as long as neither removes something from his area to his friend's area. ”

“ The Law of Four Amot in the Public Domain
דין ארבע אמות ברשות הרבים
Paragraph 1 - Every person has four amot in the public domain in which he may carry. We measure these amot by his own forearm ("amah") . If his forearm is small, we allow him four average amot, which are each six tefachim (and see below chapter 396) .
סעיף א - כל אדם יש לו ארבע אמות ברשות הרבים שיכול לטלטל בהם ומודדין לו באמה שלו, ואם אמתו קטנה, נותנים לו ארבע אמות בינוניות של כל אדם, שכל אחת מהן ששה טפחים (ועין לקמן סימן שצ"ו) . ”