The Image Size command in the Image menu allows you to alter the size of the open image.
In the "Image Size" dialog there are several ways to do this:
Type the new dimensions into the Width, Height and Resolution boxes in the Image size group box. Note that just above this the amount of space needed to store the image is displayed. The original measure is recorded in brackets. You can force the dimensions of the image to change in proportion by selecting the Preserve Ratio box, that is, typing a figure in Width will automatically cause the figure in Height to change in proportion.
The Scale group box allows you to select from one of the displayed fractional scales: 1/4, 1/3, 1/2, 2, 3.
The Interpolation group box allows you to determine how the image will be affected when the size or resolution is changed. Since changing the dimensions of an image results in pixels being either added or removed, there needs to be a method of working out what color values the new pixels will have. Each of the interpolation values is used to spread color over a given range of pixels, in an attempt to maintain the appearance of the original image. The available interpolation values are: Nearest Neighbor, Linear, Quadratic, Cubic, Quartic, Quintic, 6th Order, 7th Order. The higher the order the better the result, although it takes more time of course.
Once you are satisfied with your settings, click on OK to effect the changes. If you would like to reverse a resizing it is best to use the Undo command in the Edit menu, although you can of course resize in the opposite direction e.g. half the image size after a previous doubling.
See: