Really hard to answer with just height and inseam. The key fit issues are going to be with your upper body. Where your elbows end up. Do you know your stack/reach #'s for your road bike fit? Or whatever #'s they hopefully gave you? That might give a clue.

Do you know what bars are going to go on it? - that's nearly as important as frame to the fit of a TT bike.

If I had to pick one just on what you give us, I'd say 58. I'm 6'3" with slightly longer inseam, and I'm firmly a 58. Just don't blame me if it ends up being wrong.

You don't have to worry about "aggressive" with a 2012 P3 regardless of 56/58. That's a long/low bike.

I am 6’3” with a 36” inseam and am currently moving from an older gen 58 to a newer 61 so slightly different geometry. I ride at about 6 inches of drop. I could have ridden the 58 with no headset spacers and 50mm of pad risers)alpha x with tilt kit and ergo cups which also add some height). I don’t see how I could have ridden a 56.

Both of the above posters are correct. A definitive answer is try difficult to give, but you’re unlikely to be ideal on a 56.

The other major factor is going to be your front end. Since you’re going in blind, make sure you have a front end setup that can be adjusted in lots of ways to help you refine your fit over time. There are plenty of inexpensive ways to do this with basic components (stem, base bar, clip ons, and round spacers under the stem), since you said this is a budget build.

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TriRig.com

What do you mean by "aggressive"?

It usually means arm rests are way down, so your head is down and back is flat or lower so if you look from the front, your body mass is minimized as possible to maximize the aero.

I am about an inch shorter than you, same inseam. I was on size 56 on P3SL and am on size 58 on NP3. The P3SL was faster but was a bitch for the 70.3+ distances. I'd go 56 but you may not like it if you start going long.

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