פירוש על כלים 17:8

כזית שאמרו – as for example, like an olive’s bulk from the dead or from the carrion that ritually defiles, and most of the measurements that are in the Torah are like an olive’s bulk.

The olive of which they spoke it is one that is neither big nor small but of moderate size the egori. Many halakhic measures are based on the olive, including many minimum measures of how much food a person needs to eat to fulfill a mitzvah. This olive is of medium size. This medium-sized olive even had a name the egori oliv.

אגורי (fit for storage, of good quality) – whose oil is stored within it (see Tractate Berakhot 39a).

The barleycorn of which they spoke it is one that is neither big nor small but of moderate size the midbarit The barleycorn is the size that a piece of a bone needs to be to convey impurity. This barleycorn is of medium size and it is called "midbarit," which means wild.

כשעורה שאמרו – a bone like a barleycorn (see also Tractate Negaim, Chapter Four, Mishnah Six).

The lentil of which they spoke it is one that is neither big nor small but of moderate size--the egyptian kind. For a piece of a dead creepy crawly thing (a sheretz) to convey impurity it needs to be the size of a medium lentil, of the Egyptian kind.

כעדשה שאמרו – a creeping thing defiles like that of a lentil, and is sustenance like that of a lentil.

"Any movable object conveys uncleanness if it is of the thickness of an ox goad" it is one that is neither big nor small but of moderate size. What is meant by "one of moderate size?" One whose circumference is just a handbreadth. This halakhah is taken from Ohalot 16:1 (a tractate which we have not yet learned). An "ohel" or tent is anything that overhangs. If a dead body or piece thereof is found under an ohel and under the same ohel there is a pure vessel, the dead body defiles the pure vessel. Here we learn that if the ohel is a movable object, it must be the thickness of a medium-sized ox goad. Its circumference must be at least one handbreadth for it to convey uncleanness.

מביאין טומאה – upon a person who carries them.

בעובי המרדע (like the thickness of an ox goad) – that is from the study of cattle, and its thickness is one finger and a third-of a finger in size, which are two fingers in smallness. And the handbreadth is four fingers in size, which are six small fingers. And it is found that its circumference is a handbreadth. For everything that is its width of a handbreadth there is in its circumference three handbreadths, but they (i.e., the Rabbis) decreed on that which has a circumference of a handbreadth, on account of that whose thickness is a handbreadth, that he brings the ritual defilement from the Torah.