You already built it or plan to? I'm a nd generation contractor and have been building for over 23 years now. Marine grade pressure treated preferably, but regular pressure treated will work. You could get away with 4x4s but 6x6 would be ideal. Depending on the length and width one at each corner should work as long as you are under 8 feet each direction. I would use 2x8 for the joist but 2x6 would work. Notch the corner post and set the outer band inside of that notch so that the outer joist rest securely on the post and then screw it to the post. Use joist galvanized joist hangers for the common joist on each end. If the deck boards are really wet from being freshly pressure treated, do not space them, but them tight and when they dry out they will have a nice space between them for water drainage.