Watch out - I was in the same position, and had to buy online due to lack of stock anywhere nearby (even to sit on/test). Looking at the geometry on the website I could fit either, but they don't give you seat tube length. I think you can work it out if you can remember your maths from school (sin something or other).
As a result, when the 56 turned up I realised I would need a ludicrously long mountain bike style seat post (annoying, looks odd, and defeats the point of getting a seat post with the frame). So I sent it back and got the 58.
Fyi, a 58cm R3SL has a 55cm seat tube (measured centre BB to absolute top). I *think* the 56 was at least 4cm less, but don't quote me. I am 6'1", with an 89cm (35") inside leg by the way if that helps. That inside leg was measured with a wooden expanding thing as part of a cycle fit - I think in trousers (pants to you septics and canucks) I'm more like 33/34" as clothing sales assistants aren't generally as persistent when measuring... I have the included Syntace P6 280mm/27.2mm seatpost a few mm below its maximum extension, which suggests at a pinch (seat forward, short stem, not ideal) I could take a 60cm frame. I have a 110mm stem on there at the moment.
I suspect they don't publish the seat tube length in order to promote their 'reach-based' philosophy. I happen to think they are generally on the right track with this, but its no reason not to provide all the info a customer needs... I think especially for those with long legs, its important to check what kind of seatpost length you will need once you have your reach dialled in. If I'm missing something can someone at Cervelo explain why seat tube length is not important?
Oh, and on the 58cm I ended up using the max spacer height on the fork too (at least for now - might drop it a bit when i get used to the bike), so if you're my size a 56 might leave you too low at the front too.