Hi Kowshik,
Here first you need to be specific for what you're searching.Recent revert of yours showing for system refresh methods.
If you want to refresh your QAS system with PRD client data than follow system refresh methods (Client copy methods).
Iam having 3 clients(100,200,300) in a quality system if delete any one client how much data will remains in that quality system
To get the size of client you could execute SAP standard reports RSTABLESIZE ,RSSPACECHECK & follow SAP Note 118823 - CC-ADMIN: Size of a client
Hope this will help you.
Regards,
Gaurav