Change pixels to centimeters in stroke-outline

Hello everyone. I apologize for my poor English. In the image outline tool I don't know how to change pixels to centimeters. Although I have selected centimeter in the general preferences, it does not apply to the thickness. How can I make that change? Os estaré muy agradecido si podéis ayudarme en eso! I will be very grateful if you can help me with that!