Data usage for WhatsApp store automatically in settings. how to get there is very simple. just follow instructions.
open your WhatsApp and open options bar like this
select "SETTINGS" option like this
now choose " DATA USAGE " option like this
Now choose " Network Usage " option like this now a new window opens up... you can see your whole data usage,
you can reset your data usage to zero value
just scroll down and click on the option of "RESET STATISTICS"
when you click on the option of reset statistics and click "RESET" like this
now your data usage statistics will be reset.