I'm searching my files but so far have only the Atomic part #, not the actual size. I'm pretty sure all those O-rings are standard sizes, but the one you are talking about may be a non-standard material. There is one at the top of the center post and a pair (hard to see) further down. I'm guessing that you are wanting the top one, as it is the most vulnerable to wear. If so, their part # is 20-2003-85 I do recall (I think) that they changed the top O-ring to a harder durometer at some point, which may have made it a bit unusual and a semi-custom part. It can be a bit of a pain to install. I don't know where you are, but an Atomic dealer might be the best source for the actual part. Servicing this should be a standard dealer operation.
Ron