The 2mm diameter screws seem quite large. When would I use this screw size?

The use of rescue screws with the Screw & Tack Kit

The 2mm diameter screws are your 'rescue' screws if the osteotomy you performed with the 1.6mm screws is too large, or if for some reason a smaller diameter screw cannot engage properly. A larger diameter screw can be used to provide more rigid fixation.