Concrete Pad

Can someone advise me the specifics when pouring a concrete pad under a home. If a home is a 14 by 70 do I need a 14 by 70 pad or do I make it 13 by 64 what is the general rule when pouring a pad under a home. Also how thick should it be 3 inches or 4 inches and do you recommend rebar in it? Thank you for your advice. Bradley D.