Originally Posted by Bah Humbug

I have a 56 S1 and a 56 P3. I also know people who did -1 sizing to their Cervelo tri bikes from their Cervelo road bikes. You need to find a good fitter (ideally a FIST fitter - look on Slowtwitch) to tell you what stack and reach you need. If they can't explain what stack and reach are, clearly and quickly, they have no idea what size you should be on. Even moreso than road bikes, tri bikes need to be the correct size and random bike shop employees can't tell you which is correct (though I can't imagine going up a size is ever correct). It depends on your proportions and flexibility.