When using canvas size you can kind of resize your document without doing anything to your image. Heres how you do it :

1. Open a image or anything that's a image, I will use a psd.

2. Then click IMAGE > CANVAS SIZE

3. A new image will now pop-up where you have to change your settings. Make sure you set it to pixels. And where I have clicked the sqare in the center you click where you want you're image to appear. And just choose the width and height right to where you change to pixels.


After that just click OK and you will be done.

Here is how mine turned out: