If you start the line tool and pick a point and draw, there is a distance readout down in the coordinates area..

ALso if you just select something, the upper right corner has a properties readout.. Selecting a line will show a length value. There are some scripts on the resources page for specialized measuring.. Curve length, a distance tool, etc.

The scale may or may not be changed.. There is an option to "scale on unit system change.. But you have to know the file.. I have a program that I save files in inches, and they always load as mm in MoI. I change the units to inchs and that scales it. I then resave it back to the other app and it loads fine. If you dont pay attention, you can get the units off and scale wrong.