image - resize, like Louise said.
But I usually adjust the file size when I am saving. It gives you the option then, and asks you what quality you want. Adjusting the quality adjusts the file size.
That will adjust the file size, but to adjust the picture size the easiest way is to crop to size.
You select the crop tool, and then under the tool bar on the top you can type in what size you want the picture to be width and height. I usually type 400 px (400 pixels) into the width, and then when you crop it will resize your picture. That is the easiest way to do it. If you only fill in the width and not the height, PS will maintain aspect ratio when it crops.
400 px gets you this width size:
Obviously, you can go bigger if you want it bigger.