Open your image in Photoshop. I am using Photoshop CS2 here but the dialog you see with the version you are using will be the same or very similar. Let me know if it is not.

As a rule it is a good idea to work with an image that is not in a lossy format such as .jpg, if only because it is a very good idea to regularly save your work and every time you save a .jpg it degrades. If your image is currently a .jpg convert it to .psd or Tiff. You can do that by going to

File > Save as

and selecting your choice from the drop down menu.

Now go to Image > Image size.