The voxel or sample size is the size of each voxel inside a 3D digital image, the physical dimensions of what is registered inside a VoXel. It can be understood also as the Sampling Distance, and is the inverse of the → Sampling Density.
The word "sample" does not refer, in this image processing context, to the object you are imaging, but to a VoXel!!!
The Ideal Sampling is determined by the Nyquist Rate.
See Sampling Density for more information.