A series of DICOM files is typically accompanied by a DICOMDIR file. If this is the case for you, the recommended way to import a 3D image would be:

Import["path/to/DICOMDIR", "Image3D"]

and it will automatically use the meta information to choose proper thickness of slices.

If you only have a series of DICOM files, import all of them to a list of images, as you have already done, extract the values of "PixelSpacing" and "SliceThickness" and pass these values as the BoxRatios option to the Image3D constructor.