Terumot, Chapter Four, Mishnah Six
1) On three occasions they measure the contents of the basket:
2) At the full time of the first ripe fruits, and of the late summer fruits, and in the middle of the summer.
3) He who counts [the fruit] is praiseworthy, he who measures it is more praiseworthy, and he who weighs them is most praiseworthy of all.
Section one: Three times a year a person should check the basket where he collects and stores his fruit, especially figs, to see how much he has gathered so that he will be able to know how much terumah to give. For instance, if he sees that the basket contains 100 figs, he will know to give about two figs.
Section two: The three periods are: 1) the time when the fruits that ripen quickest have ripened. These fruits are largest and the basket wont be able to contain very many of them. 2) The late summer fruits. These are smaller and the baskets can hold a larger quantity of them. 3) The middle of the summer, when there is a lot of fruit but they are of average size.
Section three: We learned in 1:7 that when one separates terumah, he does not separate them by measuring that which he takes out, but that he can separate from a pile of fruit that has been measured. Our mishnah teaches that there are three ways of finding out how much fruit is in the basket and that some ways are better than others. The first way is to simply count the fruitthis is better than not measuring at all and is praiseworthy. The second way is to measure the containerthis is more praiseworthy. The final way is weigh the basketthis is the most praiseworthy of all. I should emphasize that when he takes out the terumah, he is not supposed to count, measure or weigh that which he is separating. He is only supposed to do so to the entire basket of untithed produce.