i dont kno wat version u hav but with mine i go to the image size option under 'image' and then i have an option to change the pixel dimensions or the document size, which are both in the same window. and with the document size i can change the units to cm, inches, mm etc
everytime i change the cm size the pixel size changes so it resizes the image all the same, and always make sure u have constrain proportions ticked so ur image doesnt get distorted
hope that helps.