Hi Ulco,
Resolution: The requirements depend on the resolution and field of view of the headset.
Aspect ratio: If you see recommendations for a 2:1 ratio for mono and 1:1 ratio for stereo, that's simply because the 2:1 left eye and 2:1 right eye images are stacked one on top of the other and then separated into left and right images by the software at run-time. Whether the images are stacked this way really depends on the viewing software.
Inter-axial separation: This is the distance between the eyes in world space. If you want to depict a realistic sense of depth this should be left at the default. However, if you use an unusual scale in your scene then you'd need to adjust this, otherwise the viewer may perceive the scene as being at the scale you actually modeled it.
In 3D movies it is quite common to exaggerate the inter-axial separation so that people get more of the "3D" they are paying for, but if you do this in VR you are more likely to make the scene feel too small.
It shouldn't really depend on the headset because it's a property of the scene. However, because of the low resolution of current VR headsets it becomes difficult to perceive changes in depth beyond a few metres away (maybe 20 metres or so).
I wrote some more about it in this thread from 2016:
https://planetside.co.uk/forums/index.php?msg=225919