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.