Hi Arun,
To know how much memory has been used by different objects in HANA database, then the easier way is to use the TOTAL_SIZE_IN_MEMORY (along with main, Delta etc) from M_CS_TABLES. This would give you idea of how much memory has been used by objects which are partially or fully loaded to memory. Please note that it give 0 value for the objects which are available in the systm, but not yet loaded to the memory.
Please see this ote:
SAP HANA Database Size - In-Memory Business Data Management - SCN Wiki
Please see this forum:Where can I see the HANA database size? | SCN
For scalabilty:
Scalability - SAP HANA | SAP HANA