Thickness and length
Sometimes the length found this way is called the ribbon length.
Warning: if the border is very rugged, the result won't be what you expect. Even a circle will be very "thin". Blurring the image may help with this problem.
Another way to measure the dimensions of an object is via major and minor axes. The results will be very different if the object isn't convex, e.g., a spiral.
Roundness is shown in Pixcavator's output table.
Some real life examples of applications of these measurements are:
See also Measuring objects.
To experiment with the concepts, download the free Pixcavator Student Edition.