CS5453 Energy Meter IC
Full scale is the maximum value that the register or channel can holdor display.

section 5.1 on page 16 says  


  The full-scale differential input voltage for the current
  and voltage channel is 250 mVP.


Current may be sensed eg with an inline shunt rsistor.
By choosing a shunt resistance or other means the current which causes 250 mV peak to occur at the current input is the full scale current.

Voltage may be sensed with a resistive divider.
The input voltage which when applied to the installed resistive divider causes 250 mV peak to occur at the voltage input for is the full scale voltage.