Think of bbox override as the area your Spine instance will render into. If scale is set to 1, it will be the same 'coordinates' as your Spine project.
In general, your Spine animation should stay centered around a point in your Spine project. If you have a jump or leap animation, it should be done 'in place' and the actual vertical or horizontal movement should be done by moving the C3 Spine object during runtime in C3.
The width and height are the size of the area that is rendered into, so it should be sized to encompass all the movements of your Spine object (e.g. large enough to capture a swinging sword.)
The offset x and y are used to center the area around your character. For example, if your character is centered around 0,0 in the Spine project, the offset should generally be -1/2*width and -1/2*height.
The main thing we usually do is experiment a little, using a scale of 1.0, then use height and width of the object itself to scale as needed.