Zoom out of the image, so you can see the grey Photoshop border. Press the tab key to hide all Photoshop tool palletes. Select the entire image (Ctrl+A) and press Crtl+T. This brings up the transform handles. From here, you can drag where the handles are, and resize the image this way. (Transform can also rotate the image, in addition to scaling it.)
Note that if you also hold down the Shift key, you can resize in proportion.
(This probably isn't the easiest method of doing so, but if you would rather not go through menus to find what you want, its easier in that sense.)