Measruing a Frame (Bike Size)

[Spyder]

This is a total newb question but I was wondering how to measure the frame size on older bikes, pre 1960. I found some good articles on google, but I have an old bike and im coming out with odd measurements when I try to follow those guidelines. So how can you tell if your looking at a 26" or 24" or etc.
Ill do an intro as this is my first post, Thanks guys

 

[CCR]

the easiest way is to measure from the back of the seat tube to the rear fender mount on the uper stays, dont know the roundabout measurement is off hand, but the 24" bikes are closer usually than a 26" would be.