I have used a circuit like that but connected to the ground side,
the resistors were R1=1K and R2=50K so the gain was 50x ,
my Vref was connected to gnd and the resistor was 0.02 ohm (1%).
so the actual output was 0,02v/A on the resistor and 0.02*50=1V/A in the opamp output.
The values depend on your input and output range as FvM said but in general the Rsen should be
as low as possibles so that you don't create any additional voltage drop but high enough to give an accurate measurement.
Alex