I don't know what the official size is when you haev them sucked all the way in.. But I know to answer one of your first questions, Yes, you can widen just one side and not the other. How its done is there's spacers (big bushings) 2 of them. To make it +1 you place hub on first... then the 2 spacers and then the axle nut.
to make it +2 , you put the 1" spacer on first, then the hub and then the other spacer then the axle nut on..
and to extend it fully, you put both bushings on first, and then the hub and then the axle nut.