How is it possible on PCBnew to measure length ?
Inspect / Measure, or click on the measure tool.
If you have a object on the PCB you can select the object “E” key to edit. You will get a window with start and end points and length. (all the data for the object)
Double plus good for the caliper tool gmc mentioned.
There are also coordinates at the bottom of the screen that follow the mouse cursor, and the “delta” part of the coordinates is reset to zero every time you depress the space bar.