Yes it will definitely help. To keep the balance right, try to install spacers both in front and rear.
Size should depend on your requirement. The best without any major compromise is .5 or .75 inch in front and 1-1.25" in back.
How about using tyres one size bigger than suggested by the manufacturer? I think by carefully selecting a sightly bigger tyre may solve some issues of road clearance without risk of any instability.
Prius ecu constantly check speed of your car through abs brake sensor (vss for prius as well) if you move to bigger tyre you speed sensor will see tyre moving slower compared to original speed this will seriously mess up things