I'm about 5'10", with proportional arms/legs, and riding a 56cm Trek (it does feel right). But I was just wondering if (based on height) it sounds about right to you guys... 5'10=56cm?

Thanks.

Sounds about right, I'm 6'-3" ride a 58cm. All in how it is sized. Go to Competitive Cyclist and run through their fit page. Great Info

it depends on your riding inseam and your torso and arms lengths and a bunch of other factors.

But I am just under 5' 8.5" (173cm) and I ride a 54cm Madone with 100mm stem. Another example is Lance (who?) is ~177-178cm (i.e. ~5' 10"?) and he rides a 58cm Madone with a long 130mm stem, if I am not mistaken.

Chances are 56cm is your frame size unless you have irregular dimensions.

I'm 6' and I am comfortable with my 58cm frame. It all depends on your boby proportion.

Measure your inseam: stand against a wall with your feet 6 inches/15 cm apart. Push the spine of a 1 inch/2-3 cm thick book into your crotch with significant pressure, and measure the distance from the book spine to the floor. Your saddle top to pedal axle should be 108-110% of the inseam measurement.

Here are several frame fit calculators.

http://www.bsn.com/cycling/ergobike.html
http://www.coloradocyclist.com/BikeFit/index.cfm
http://www.sheldonbrown.com/harart-frames.html
http://www.rivendellbicycles.com/frameinfo/Frame_Sizing.htm
http://www.peterwhitecycles.com/fitting.htm

They give you good estimates of sizing, but as others said, it depends on your proportions.