You can draw an Annotation ROI and set the region where you want to edit your segmentation:
- Click down-arrow on the Place button on the toolbar, select ROI, click in the image, adjust the ROI size
- Go to Segment Editor, click “Specify geometry” button (next to Master volume selector)
- Choose the ROI node as source geometry, set spacing values, click OK
This method may require less steps than what @muratmaga suggested above but has the disadvantage that the spacing and origin values of the segmentation will not match exactly the master volume’s (that you might find confusing).