|Sa` is a certain measure that equals 4 mudds (a mudd equals a handful of an average man). The contemporary equivalent weights of Sa` differs according to the stuff which is weighted. For example, a Sa` of wheat equals 2176 grams, a Sa` of rice is 2520 grams, a Sa` of beans equals 2250 grams, etc.
Therefore some scholars are of the view that the criterion should be the measure, not the weight for there are kinds of food which are heavier than others. But I think this is the case if the equivalent weight of a certain kind of food is not known. If there is no available measure of weight with the person, then he should pay 4 mudds.
Nowadays, it is not that problem because ministries of religious affairs in Muslim countries and mosques and Islamic centers in Western countries announce the value of Zakat al-Fitr every year. Anyhow, this is the obligatory amount that every Muslim should pay. It is better and recommended that one pays an extra amount, particularly for those who are wealthy, for they will be rewarded for it.