Originally Posted by Trakhak

You can quickly learn to eyeball older level-top-tube steel brand-name bikes, originally bought from bike stores. In the photo of the Miyata, look at the back of the head tube where it's visible between the top tube and down tube. There's maybe a half-inch of top head tube, if that. That means the frame was probably sold as a 19" size. (At that price level, the sizes would have been in 2-inch increments, and 19" would have been the smallest size.)

So if you see a photo of a bike with about 2 or 2 1/2" of top head tube showing there, it's likely a 21" frame; if about 4", a 23" frame, and so on. You can usually guess pretty closely.

Upper-end bikes are trickier, since they were often sold in 2-cm size increments. Still, look at head tubes and you can do a reasonable guesstimation of whether a bike is in the ball park for your size.