The 'tutorials' board is not the place to ask questions, but i'll answer yours anyways.

1) Open the image.
2) On the top, click on 'Image'.
3) Click on 'Scale Image'.
4) Now put in the variables (desired width and height measures) and press 'Scale'.

NOTE: Resizing takes away the quality from an image, so I wouldn't suggest doing so. Especially if you plan to wear them as textures, but that's your choice.