Specify Length of Line?

How do specify the length of a line.
Sometimes the window for entering a length shows up on the top bar but 90% of the time it does not show up.

How do I open the window for entering "length of line?

Thanks in advance for your time.
Warren

The top bar is the "Tool Options" bar and it's contents varies according to the the active command. So length of line is only asked for when it makes sense.