1) there is no minimum size, but 15 x 15 meters should be usable. I recommend to set a bigger area in any case, because it will be hard to keep model always inside.
2) You can change the response with: GeoLink - Horizontal Correction Gain, Cyclic Ring. The first one will make corrections more or less gentle. The second one will limit how big cyclic pitch can be used for stopping when reaching the boundaries.