The $PACKSET directive takes a numeric argument of 1, 2, 4 or 8. This number determines the number of bytes used to store a set: The compiler rounds the number of bytes needed to store the set down/up to the closest multiple of the PACKSET setting, with the exception that 3-byte sets are always rounded up to 4-byte sets.
Other allowed values are FIXED, DEFAULT (or 0), or NORMAL. With these values, the compiler stores sets with less than 32 elements in 4 bytes, and sets with less than 256 elements in 32 bytes.