Building Deck question

I have some material and plan on building a 20 x 14 ft deck about 4ft off the ground. The joists will be 14ft 2 x 8's, the posts will be 4 x 6's on a concrete pier 30" deep x either 10 or 12 diameter (which size should I use?), the beams with be two 2 x 10's. I will have 3 sets of beams. First row parallel with the house will be 3 posts, middle row 4 posts, outer 3 posts. I know timber strength determines alot with the joists. The 2 x 8's are stamped No2 Prime, Tp 283, KDHT, SYP. I dont know what this means. My question is will the 2 x 8's be OK and what size diameter should I make the footing that will be 30"deep.. Frost line here is about 25". I have included a pic of the design.. Please advise

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