Like you said, ParaView does not directly support units. But you can adjust the size and location with the transform display properties. In the Properties panel, turn on the advanced properties (that gear icon near the upper right) and scroll down about halfway until you see a group of controls labeled Transforming as shown in this screen capture.
These controls let you adjust the size and position of the object in 3-space. You can use the Scale parameter to adjust the size of each image. I suggest setting the Scale to the voxel size of each respective image. So for your first image, set the Scale to 0.5, 0.5, 0.5 and for the second image set the Scale to 0.273, 0.273, 0.273.
If you need to move the images around to line them up, you can do so with the Translation parameters.