R. Mordechai Willig calculates the preferred minimum amount of matzah to eat, i.e. the size of a ke-zayis (link - audio). This is, by necessity, based on approximations and averages. Here is how I understand his steps:
- According to measurements in Israel, the average size of a contemporary egg is 50 ccs. (The majority of classical authorities do NOT double this size.)
- Measurements have shown that an egg's volume decreases by 10% when the shell is removed, which leaves us with 45 ccs.
- The strict view is that a ke-zayis is half of an egg's volume, which is 22.5 ccs.
- Measuring the required volume of a matzah should be done on the matzah as it is, not by crushing it into crumbs. The weight of 22.5 ccs of water is 22.5 grams. Click here to read more
- Experiments show that the equivalent volume of matzah has half the weight of water. This means that a ke-zayis of matzah weighs approximately 11.25 grams.
- 11.25 grams is about 0.4 ounces.
- There are on average 7.5 handmade matzos per pound (16 ounces) and 15 machine-made matzos per pound, which means that an average handmade matzah is 2.13 ounces and an average machine-made matzah is 1.07 ounces.
- Therefore, a ke-zayis is less than 1/5th of a handmade matzah and less than 2/5th of a machine-made matzah.
For those of us who have the custom of eating horseradish as marror, this might help. For a few years, I would not recite the blessing over marror because I did not think I would eat a ke-zayis. Even though now I can eat plenty of horseradish, I think these measurements might be useful sometime in the future.