Based on this image in your other post, it seems you are using scans of a printed film, so probably the image data is not calibrated. You may be able to get a rough idea of the size based on appearance of the various tissue types. Be sure to confirm that the sizes of the voxels have been accounted for when calculating the volume (e.g. you should know the spacing between the slices, perhaps by looking at the text annotations, and you can adjust for the pixel spacing by measuring a known distance in the slice plane, such as a ruler printed on the film).