This is bit off-line topic(DBA Track) for sizing process in new EBS installation. Line managers, infrastructure heads, IS team, DBA and Implementation Leads must have to understand some key points for sizing as:
- Sizing is a complex, iterative process and must be treated with care during the implementation.
- It should not get conflicted as Sizing a mere calculation rather it a process.
- Sizing is a project in its own right (PC, network, middle-tier and database-tier). Network is often forgotten in sizing projects.
- System sizing and architecture goes a long way in determining the TCO of the software
- It potentially means ability to grow vertically(Module wise) and Horizontally (Data)'
- Customer, hardware vendor and the consulting patner should work together to find the best sizing strategy.
- Your database Size should be based on benchmarks and collective experience of Oracle, H/W vendor and knowledge gained from existing customers.
- Sizing process should not be confused with capacity planning, as both are different parameters.
- Scaling & load balancing requirements can influence the architecture landscape
- Validation of assumptions is key.
- Predicting the batch processing load should not be ignored during an ERP sizing study. Batch loads may be a more important a factor than online requirements.
Factors which decide some key drivers for sizing are:
- Number of named users
- Number of concurrent Users
- Number of Concurrent Manager (CM) Process
- Business Event
- Business Intelligence (BIS)
- Number of years online data retention
- Business volumes
- Platform & H/W preference
Those who are looking some more granular details for sizing, there is one nice document from Oracle.
- Sizing_EBS_LongVersion.ppt 
- or download
- Note 399362.1 :Oracle Applications Release 12 Upgrade Sizing and Best Practices