I searched on the internet, there are a few  Queries can be used for database sizing information:

dba_data_files: 

dba_segments:

dba_extents

Can anyone advice which will be more accurate for data growth calculation? and how should I work out the actaul data growth in a period of time?

Very much appreciated!

Can you also explain on dba_extents?

Thanks!  It probably explained why I got different results by using dba_extents and dab_segments.

I believe starting from 10g Oracle has provided us with a feature to measuring Database space growth. This delta information can be queried using views DBA_HIST_SEG_STAT by collecting segment growth periodically. There are a lot SQL code you can search on-line.

One interesting thing I find is that some article said Oracle provides time-series segment statistics for tables and index in the 10g Automated workload Repository tables (AWR). But I really got no space information in my sample AWR report. It has only segment/tablespace IO stats. Maybe many AWR collection method existed I need to research.