patch antenna dimansion for special resonance frequancy

i want to design a simple patch antenna which resonance at 3.5 gig . how i can find appropriate dimansion? by try and error?
i fin that patch:32*32& substrate :38.3*38.3 width:1.4 ; er=2.35 resonate at 3 gig :< plz help is there any formula for design?

There are many formulas for design. See google. I also would avoid making a square patch - this makes life harder, as you can't be sure which mode you are exciting.