The progress bar is designed to simply display the current % complete for a process. The bar is coded to be flexibly sized through CSS and will scale to fit inside its parent container by default.
This is a determinate progress bar, meaning that it should only be used in situations where the system can accurately update the current status complete. A determinate progress bar should never fill from left to right, then loop back to empty for a single process — if the actual percent complete status cannot be calculated, an indeterminate progress bar (coming soon) or spinner animation is a better way to provide user feedback.