At 5'10", 33 inseam...a 56 is probably correct for a road bike...you may want to size down if you're looking at a tri/tt bike.
I'm 5'8", 30 inseam and my road bike is a 54 but my tri bike is a 51.
EDIT - Maybe this will help.
I'm pretty sure that my Roadie (2003 Specialized Allez Pro) has a 54.8 top tube and my tri bike (2006 Cervelo P2 SL) has a 53.5 top tube at 75 deg that really works out to 51 if you use the forward seat tube angle (78 deg).
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Last edited by cjbruin; 03-27-08 at 07:43 PM.