There is a PDF document with a table for DataONTAP 7.3 (sic!) stating that read_data has a size of 8B. So probably the size is 8 Bytes. But it would be nice to get this confirmed and updated from NetApp!
Douments name and chapter is: NetApp® Unified Storage Performance Management Using Open Interfaces, Appendix 1 - Performance counters in Data ONTAP 7.3