I'm in the market for a used cyclocross bike to use as a winter bike, and so I've been browsing craigslist. I generally ride a 56-58 cm road bike frame, so I turned down an otherwise good deal because it was a 52 cm frame. The seller told me that a 52 cm cyclocross frame was comparable to a 54 cm road frame.
Is that statement true? And if it is true, how is that so?
The first and foremost measurement is effective toptube length. There are, of course, other deciding factors in how a bike fits, but effective toptube length plays the biggest factor in whether a bike is going to fit or not.
Bikes are still measured in terms of seattube length which made some sense when many bikes had the same toptube length regardless of size These days that measurement has virtually no meaning with the varying degrees of compact geometry that most manufacturers implement.
To answer your question, cyclocross bikes don't fit a size bigger than road bikes as a rule. Some folks like to run a size smaller for a cross bike, some a size bigger. Your safest bet is to go for approximately the same cockpit length that you've got on your road bike. If you ride a 58 that has a 58-ish toptube, chances are a 52 even with a 54-ish toptube is going to be too small.
Bicycle sizing may vary by bicycle type, region, manufacturer, and material. And measurements typically are of a length of the frame (usually bottom bracket to top tube: How do I measure the size of my bicycle frame?). So it would not be surprising if this statement was true.
Depending on the geometry of the frames, two bikes may have different "sizes" but actually fit the same size rider well. It is possible that cyclocross frames tend to have a smaller measured sized.
Here is a post that goes in depth... and is ultimately inconclusive: http://www.bikeforums.net/archive/index.php/t-138316.html
You're doing much more in terms of manoeuvrability on a 'cross bike so you tend to go for a less aggressive geometry which in non-custom frame translates to generally a tiny bit smaller. So if you ride a 52/53 standard frame on road you'll probably be looking at 51/52 for cross.
The bottom bracket on a cross bike is 3 to 4 cm higher so a 58 road is the same as a 54 or 55 cross bike. With the higher bottom bracket, the seat tube is shorter. They raise it for off road clearance.
