3D View > Voxel Size

The Voxel Size group of the 3D View ribbon has controls that allow you to define the size of the voxel -- the three dimensional 'pixel.'  In effect, these controls allow you to apply gains to the X, Y, and/or Z plains in order to artificially extend them or make them smaller.  For example, you can make the Z plain appear twice as deep by entering a value of 2.00, and you can make the X plain appear half as long by entering a value 0.50.  

The values displayed are the last ones used. If the image does not have a spatial calibration, and no system calibration has been set, the values are 1, 1, and 1 by default. If the image is spatially calibrated, the X and Y values and the units are taken from the calibration.