Mishnah for Eruvin 7:8

כַּמָּה הוּא שִׁעוּרוֹ. בִּזְמַן שֶׁהֵן מְרֻבִּין, מְזוֹן שְׁתֵּי סְעוּדוֹת לְכֻלָּם. בִּזְמַן שֶׁהֵן מֻעָטִין, כִּגְרוֹגֶרֶת לְהוֹצָאַת שַׁבָּת לְכָל אֶחָד וְאֶחָד:

How much is the (required) amount (of the eruv)? When they are many, [eighteen men or more] — enough food for two meals for all. When they are few, [fewer than eighteen] — the size of a dried fig, [the amount for liability] for carrying out on Shabbath for each one. [For one who carries out food on Shabbath is not liable for less than that amount.]

Mishnah Kelim