A voxel is a 2D pixel in 3D. This term is the contraction of VOlumetric piXEL, it’s a simple 2D pixel but seen in 3D. The voxel size depends on the magnification and is related to the distance of the sample from the x-ray source and detector.
Voxel size and spatial resolution are often confused. The voxel size is the size of a pixel in 3D. Voxel size isn’t related to the achievable spatial resolution of the CT system. For example, if the x-ray spot size (focused x-ray spot from the source) becomes larger than the chosen voxel size, the spatial resolution of the system becomes poorer.