Determines correct paintbrush size based on the diameter provided by the user in millimetres.
var sf = VQ.sliceViewer().getScaleFactor();
var vX = VQ.dataManager().voxelSizeXY();
var vZ = VQ.dataManager().voxelSizeZ();
var pixPerMm = vX / sf;
VQ.mainWin().setViewMode("Slice View", "3D ROI Tool");
VQ.getWidget("tabWidget").setCurrentIndex(1);
var bullyRadius = VQ.getWidget("bullyRadius");
var mm = VQ.getDouble("Brush size", "mm", 3.0);
bullyRadius.setValue( mm / pixPerMm +0.5 );