According to a few forum posts (like https://developer.vuforia.com/forum/general-discussion/image-target-width), the width entered for the target can be arbitrary and only affects the scale used in Unity. However, when in stereo, it seems like the width entered affects the distance the depth where the left and right eye overlays converge. I don't see official documentation about what units to use except when uploading it says "The size of the target should be on the same scale as your augmented virtual content. Vuforia uses meters as the default unit scale.".
I have a target that's physically 16 x 16 in. Initially we used an arbitrary unit of about 2.7 and it seemed to be relatively comfortable to use in stereo. However, we recently tried to change the units to meters (so the new unit is 0.4) but now the virtual objects in the left and right eye are much more separated and it is not comfortable to use.
I have 2 specific question:
1) Is the width supposed to be arbitrary (in which case the behavior I see is a bug) ?
2) What's the approximate delta between virtual objects rendered in the right and left eye supposed to be in stereo if you have an object that's directly on the target and looking from about 2 feet away? (In my case there's a 0.5in delta which seems to be too large)