You can do this with the Segment Editor module.
Add a region of interest (ROI) widget that covers the area where the volume will be (on the toolbar, click down-arrow of the place icon then click in the center of the region and at a corner) and set the desired voxel size:
After this, unfortunately, you need to choose a volume as Master volume, because several effects of SegmentEditor requires presence of a master volume. It can be any volume, for example you can download MRHead image using SampleData module. We’ll figure out some way to not require this step (maybe create a blank volume), but right now you need to do this.
Then you can start painting using Paint, Draw, scissors effect, etc.
Once you are done, you can export your segmentation to a labelmap volume by right-clicking on the segmentation node in Data module and choose “Export visible segments to binary labelmap”.