For the Cervelo, it may not be possible to go bigger than a 25. (for that matter it might not even be possible to use a 25 depending on the brand). Remember, that the wheel and tire are going to flex in corners and you are going to need a little more clearance than it appears you have at rest. Any rub marks on the fork or frame at all are a clear indication that you have gone too large.
I ride at 225 on my Leader bikes TT frame and use the CXP 33 or CXP 22 combination with a 23mm Kenda Kalliente Iron Cloak (kevlar antiflat belt at an ultralight 195 grams). I run them at 110 lbs and they hold the weight ok. My wheels however are custom (I build them) 32 spoke wheels with DT double butted spokes and spoke washers at the head to make the spoke fit in the hub better. They are radial spoked fronts and half radial spoked rears which I believe hold up better to the 225 lbs.
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