Siman 486
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א שִׁעוּר כַּזַּיִת, יֵשׁ אוֹמְרִים דְּהָוֵי כַּחֲצִי בֵּיצָה. הַגָּה: וְיֹאכְלֶנּוּ בְּיַחַד וְלֹא מְעַט מְעַט; וּבִירָקוֹת צָרִיךְ לִמְעֹךְ חֲלַל הָאֲוִיר שֶׁבֵּין הַיָּרָק, וּלְשַׁעֵר שִׁעוּר הַכַּזַּיִת בַּיְּרָקוֹת עַצְמָן וְלֹא בָּאֲוִיר שֶׁבֵּינֵיהֶם.
1 The Measurement of a Kezayit (Olive-Sized Portion): Some say that a kezayit is equivalent to half an egg. Rema: One should eat it all at once and not little by little. Regarding vegetables, one must compress the air spaces between the leaves and measure the kezayit based on the actual vegetable itself, excluding the air between them.