Since compression factor and other hardware requirements are dependent on the scenario, there are different sizing rules and SAP notes.
General Sizing (SAP Note 1514966):
This describes the sizing of the SAP HANA as it is used e.g. for replication of ERP data coming from an ERP system. In particular, these rules must not be used for sizing BW on HANA and Business Suite on HANA systems.
Sizing for BW on HANA (SAP Notes 1736976 and 1637145):
In the SAP Notes listed above a detailed description of the sizing of the various components is included. Besides, there is a sizing script available that supports sizing for a migration.
Sizing for Suite on HANA (SAP Note 1872170):
The corresponding SAP Note describes how to implements a report to estimate the memory space requirement for the database tables of Suite on HANA systems.
Non-active data concept for BW on HANA (SAP Note 1767880) and Nearline Storage Solutions:
Large BW systems contain large amounts of data that are no longer or rarely actively used but that should remain in the system (historical data, keeping data for legal reasons, and so on). This data is called non-active data. An implementation for BW on HANA allows to displace non-active data in case of main memory bottenecks leveraging a last-recently-used concept. This concept improves main memory resource management, which has positive effects on hardware sizing for a large amount of non-active data. For more information about this, see also SAP Note 1736976. Besides, nearline storage solutions could be used to store “cold data”, which can additionally help to reduce the memory amount.
SAP HANA Smart Data Access (SAP Note 1879294):
SAP HANA smart data access enables remote data to be accessed as if it was stored in local tables. Since the data is not copied to SAP HANA, it does not need to be considered for the main memory sizing of the SAP HANA server.