Geometry on different bikes, and how the Manufacturers measure, is going to play into this a lot. I have a 59cm Merlin Extrlaight, a 23" (58.4cm) Schwinn Paramount, a M/L Giant TCR Team Advanced, and a 56cm Cervelo P2C TT bike.
And even though there is a 3 cm range, they all fit.
The significant thing is they all have almost the same length TT. The Merlin's TT is 57.5cm; The Giant's effective TT is 57cm; The Cervelo's effective TT in the 75 degree setup is 57.3cm, and IIRC the Schwinn is also57 cm.
All that said, my Cervelo is nominally one size smaller than my road bikes.
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