If you have an object that is not the size you want it to be then edit its group/instance, and use the tapemeasure tool to pick two points across it [typically two vertices] - let's say the VCB reads '~456,7mm', but you want it to be say '18mm'... then simply type 18mm <enter> [as you would for many of the native-tools that will take VCB inputs] and when the dialog asks if you want to scale everything you must answer 'Yes'. If you now pick the same two points the VCB should now read '18mm'.
S