it will "eat" volts so use as low a value as you can (trade-off between signal resolution and what volt's you can afford to lose)
The correct place to put the "sense resistor" is actually in the load (ie in series with the machine" since then you will also be able to sense the free-wheel current (EXTREAMLY IMPORTANT if you have a PI-control and periods where no switching occurs => freewheeling, since the current-loop 'I' part will wind itself off and cause nice big current-spikes at the next switching event)
You don't seem to have that problem in this setup so put the low-value resistor on the negative side of the power-rails. That way can easily reference w.r.t. 0V