Answer:
Rebar is measured by length (so inches or mm) and is usually followed with by weight.
There are various sizes (diameters) of bars. Each size has a weight per length than you determined weight by length X barsize weight per length. It can be done in metric or imperial, just have the appropriate and corresponding chart that is relevant. For projects, the total rebar weight of the job is usually what is used to tell overall scope of project size.