The size of image elements influences significantly the quality of the image. A small voxel size increases the resolution but produces high oscillations of a signal. Large voxels produce homogeneous images, however, with low spatial resolution. The size of the voxel is always a compromise between suppressing signal noise which requires larger voxel size, and covering small imaging details which requires smaller voxel size. For the in-beam PET scanner BASTEI the size of the voxel can be enlarged for typical irradiation fields from the standard (1.6875 mm)³ to 2 × 2 × 3 mm³ without compromising the image quality. The images reconstructed with larger voxel are less noisy and still contain enough information about activity in small cavities and range deviations. Reconstruction is two times faster for the 2 × 2 × 3 mm³ voxel, as compared to the standard (1.6875 mm)³.