I don’t think it effected the import or export size… Anyone want to guess what changing to inches or feet or miles or whatever effects so I can go and test to see if that is what it is? I thought it might change the entire grid itself… but no.
Changing the “Scene -> Units” is only a reference for the scene units. It works in conjunction with the export option “unit of the file”, which by default is set to “None”(for .obj files). So changing the “scene -> Units” by default will have no effect at all on importing or exporting .obj files.
If in the export options, you set the “Unit of the file” to a specific unit type, then that can cause the import/export of that file type to be converted.
For example. If you have the “Scene-> units” set to mm, and the export option for “unit of the file” set to cm, on exporting a 10 unit cube, it will be exported as a 1 unit cube (10mm = 1cm). It is similar on importing. If you have the export/import “unit of the file” set to mm and the “scene-> unit” set to cm, then a 10 unit sized cube will import as a 1 unit sized cube.
To try and answer some of your other questions.
The “scene-> units” does not affect the grid size. To change grid size go to “Edit -> Preference editor” -> “User interface-> Grid”
There is no snapping to grid. Hexagon uses the “Rulers”. To reset the rulers to world 0,0,0, make sure you can actually see the world 0,0,0 and click on (while no tool is active) the “XYZ” button top left of main view. If the world 0,0,0 in not in current view, the rulers will be set to center of current view.
(Note: When using the “rulers”, while they are active, press the numpad “+/-” to increase or decrease the snap distance of the rulers.)