Measuring Bikes

I am completely clueless as to how to measure a bike I am trying to buy on ebay. There are two that I am looking at 51cm and 56 cm. Does anyone know what this means, i.e., do I measure with the tires on or is it just the frame or what?

Any help would be appreciated.

Ann

I found my answer under another thread!

THANKS!

If this is your first bike, and you are planning on riding comfortably, I would strongly suggest that you NOT buy a bike off of ebay, and instead get good service from a local bike shop (LBS). Proper fit is pretty important, as you can probably tell from these forums.

Best of luck!