Hi, Saikat - the reason why Pega sizing is different is that Pega Platform/Application sizing is focused on work to be done in Pega, but the work by Robotics is actually done with customer's legacy applications. Thus, different sizing principles apply.
The sizing of Robotics work is contingent on developer team experience, results of application discovery through interrogation, and customer's testing cycles. Thus, the sizing is usually done by the developers, who will be responsible for the project, in the context of the customer environment.
These are the key criteria that affect sizing:
1) Number and type of applications in scope of the automation
2) Number of core applications - meaning applications where the critical path business process is automated the most
3) Number of variabilities and sub-processes in scope of the automation - for example when an automation is required to do the same overall thing for multiple products, but each product has to be processed a bit differently
4) Complexity of exception handing - usually RDAs will have lower exception handling complexity because you can always hand over exception resolution to a user, but for RPAs you need to code exception handing in addition to the happy path to ensure that the automation is stable when running unattended.
5) Number of integrations with any enterprise databases, .Net components, webservices, or Pega Platform.
6) Availability of test data and level of dedication from SMEs to participate in testing of the automation - this can swing the calendar time for automation development by several weeks.
For Robotic sizing, you should need to look at three components: Number of transactions in a 24 hour period, availability of the applications that the robots are working with in that period and finally the speed of each transaction or number that can be completed in an hour. For example, 60 minutes per hour at 5 minutes per transaction would allow you to completer 12 per hour. If you have 1,000 pieces of work @ 12 per hour * 20 hours of availability, you would need 5 robots. 1,000/20 hours = 50 per hour to complete / 12 per hour (for 1 Robot) = 4.166 robots needed.
Good point, Preston! There are actually three sizings applicable to Robotics:
1) Development effort sizing, which is in my reply.
2) Bot capacity sizing for the number of machines where the bot will run, which is your follow up comment, and
3) PRPC sizing for maintaining case data (assuming RPA is built using the best practice of utilizing Robot Manager on Pega platform to farm out work to RPA bots). Standard PRPC sizing based on case volume will apply.