There is no rule of thumb for sizing the hardware as it all depends on the capacity of modules you are trying to implement.
First thing I would suggest is to review with business on few of the key questions:
1. How many users and transactions your business is trying to process via each module? - Total capacity
2. How many users or transactions your business plans to process concurrently? - Concurrent capacity
This will give you a fair idea of what should be the capacity of the modules you are trying to provision. Once you have that knowledge then you can go to next phase of sizing the hardware.
Here are go with few of good notes which will give you an idea for sizing:
How Determine The Hardware Specification : CPU, DISK, RAM (Memory) For E-Business R12.1.x Installation? (Doc ID 789438.1)
This should give you fair idea. Another often overlooked side is Database. Remember while sizing you Application Tier you will also have to increase the resources on database tier so that it can handle the additional load from new modules. Good Luck !