deckpost
I've read that deck posts have to 4ft underground to get past the frost line. With a 6' x6 [ed: not sure if this mean to refer to 6×6" or something else?] which means you only have 2ft above ground.
What if the deck needs to be in the air 5ft — would I have to get 10ft post?
Without seeing the plans and what kind of precast piers you have - I would say that I'd pour more concrete, with some rebar drilled and epoxied into the precast piers and the new concrete poured over to join them both.
Then set stand-off post bases into the new concrete for the PT posts.
It works for pole barns, it will work for your deck. However, be careful of the quality and/or treatment of your PT lumber - I've seen pressure treated lumber rot off in the ground despite being pressure treated. I think the thicker sections (such as 6x6) are rather difficult to get throughly treated all the way through. I've seen some pole barns use 3 2x6 laminated (though that was ALSO a trick to be able to have PT in the ground and non-PT above the ground, by having a staggered splice in the layers after it was above ground.) Presumably there are more carefully done ones that are "post or piling" grade and less well done ones that are "landscape timber" but in many cases there's just one type in stock, take it or leave it - so you might want to look into it more and figure out when you should leave it and shop elsewhere, or special-order something different.
Best Answer
You should use concrete footers in concrete form tubes buried in the ground deep enough to meet the code in your area. ( below frost line ). You need to use the apropriate size for your soil and the weight of the deck you are building. The concrete footer would extend above the ground by a few inches, you would then use a Simpson post bracket anchored into the concrete to set your post on/in. This keeps the bottom of the post from sitting in any water that may accumulate on the top of the footer.
The height of the post will be the distance from the top of the bracket on the concrete footer to the bottom of the beam that caries the deck joists. You will need the determine that length once you you have the footers in place and you have the height of the deck joist and the dimensions of the beam you are using.