In order to measure the volume of any part of body, you can label that part and then go to:
“Welcome to Slicer” → Quantification → Label Statistics
then, set the original image and the label that you provided. Next, you can see the volume of each label in voxel size,…
In order to label a brain, several tools have been provided in “segmentation” as well as other modules in “Extension managers”. See also,
https://www.youtube.com/results?search_query=slicerbs