deck footing size ?

Plumber here-looking to reinstall 2nd story deck support. 16'x 14' wood deck with wood rails, supported 1. attached to house. 2. 4x4 posts (rotting at soil level) and 10" x 2" doubled up beam 14ft long on edge 8ft off ground. . Not trusting self so asking for someone who been there done that. My plan is support deck temporary both ends,even raise a little if needed. Backhoe auger is 8" , 54 " deep maybe few times to get desired footer size. What is proper size footer round? decks .com or some chart says 18" round each for 2 posts. Lastly most important---saw home depot plastic 16" footer pad for bottom of hole---6x4 rests on top. No cement required just backfull post (6x4 pressure treatedx 12 ft) and level. Cement tubes, cement tub with smaller column? What is best fastes proper way? Thanks Mike in Detroit where frost line is 48". Thanks