Package size is the keyword used in ABAP statements while doing large data extractions. The package size keyword helps to create an internal table large enough to fit well into the memory. It gives the important advantage of being able to extract data from a reasonably large number of records at a time using the "SELECT" statement, thus minimizing the performance problems that may be encountered with the "SELECT ENDSELECT" statement in ABAP. Package size is recommended in cases where the data processing needs to be finished in limited time and when there is a lack of memory.
Techopedia explains Package Size
The package size number, "n," one should use depends on the size of the table contents being transferring and also on the size of the rollback area. Database administrators can give more details on this. The syntax of a package size statement is: Here "n" should be greater than or equal to zero. Otherwise, it will cause runtime errors. Also "n" should be of integer type and the usual conversion rules are also applied to "n". Package size "n" results from extracting data in packets of "n" lines, unlike the usual line-by-line data extraction and placing it into the internal table involved in a SELECT statement query.