Thank You David Thomas! I thought it would be better to add a tutorial. And i can do it now. Thanks again!
If you have a sketch like that and you want to measure it, so start with the next step.
Click on the arrow on the top, then Tools, then Measure... (You should be editing your sketch)
You will see a ruler that you can move with your mouse. Click on the curve when measure tool is selected.
You will see the Arc Lenght and Chord Lenght in a box.
You can use that tool for both 2d and 3d sketches.