Hi tibuch, thanks for the help!
I changed the voxels how you said, but although it now says it has a resolution of 768x768, the image is now stretched.
Here's how it looks in imageJ:
Here's how it looks in other software (3D slicer):
I don't understand why it automatically comes up with the right size on other software but not in imageJ. Unfortunately I can't work out a good workflow for making lots of length measurements in these other software packages, so I think it would be good to use ImageJ.
I want to be able to accurately measure the gaps between the rocks in three dimensions, so it's important to me that 1) the sizing is accurate and 2) I can easily/quickly be sure I am measuring at the same point in the three directions (hence why I would like to use orthogonal view for measuring as I can place myself with the crosshairs).