This Mishnah states that a piece of a cloak (which is a material approximately the thickness of matzah, or less), which is the volume of an olive, is less than three-by-three finger lengths. Unless you're going to say that people back then were much bigger (which indeed, some people do say), then we see that a kezayis of matzah is significantly smaller than about a quarter of a letter/A4-size piece of paper.טלית טהורה שהכניס ממנה שלש על שלש לבית טמא נטמאה. וטמאה שהכניס ממנה אפילו כזית לבית טהור טמאתו:A pure cloak of which a three-by-three [finger-length] piece entered an impure house becomes impure. [If it was] an impure [cloak], if he extended even the volume of an olive [of it] into a pure house, it makes [the house] impure. (Mishnah, Nega'im 13:8)
Here is a list of other posts relating to this topic:
Matzah/Maror Chart for Rationalists - so that you, too, can have a chart!
The Popularity of Olives - exploring why this paper is so popular and yet hated by some.
Why On Earth Would One Eat A Kezayis? - discussing the strange notion that one should aim to eat a kezayis of matzah on Seder night.
The Riddle of the Giant Kezayis Defense - wondering why many people would not accept that a kezayis is the size of an olive.
Maniacal Dishonesty About Olives - exposing an error-ridden critique that appeared in the charedi polemical journal Dialogue.
It's Krazy Kezayis Time! - discussing the view that one should eat a huge amount of matzah in a very short time in order to fulfill all opinions.
The Kezayis Revolution - announcing the fabulous sefer by Rabbi Hadar Margolin, which presents the same arguments that I brought but in a more yeshivish manner. He also brings an astonishing array of evidence that many recent charedi gedolim likewise held that a kezayis is very small, including even the Chazon Ish! Best of all, the entire sefer can be freely downloaded.
The Biblical Museum of Natural History:
First, there are lots of tours over the next few weeks, including before Pesach as well as Chol HaMoed. But they are rapidly filling up, so if you'd like to come, book your tour as early as you can!
Second, we are really looking for people who support our goals of educating the entire spectrum of society about the relationship between Torah and the natural world, and who want to be part of our mission. To join the museum as a patron, please see http://www.biblicalnaturalhistory.org/support/ for details. We can now arrange tax-deductible donations in Israel, the UK and Canada, as well as the US. For easy online donations, please click this link. Thank you for supporting our mission!