This is a common need in photoshop, to change the file size to a prescribed size, either larger or smaller. This is very easy to do, but care should be taken to adjust it only once from the original image to the finished size.
The main dialog to do this with is the Image Size Dialog. This is in the "Image" Menu, and it is found by selecting "Image Size..."
This changes the overall size of the image. It is divided in two portions, the pixel dimensions, and the document size. The DPI is really not important, as the only real thing to focus on is the pixel dimensions. To the right is the unit of measure which can be Inches, Pixels or Percent among others.
- Constrain Proportions, when checked maintins the original proportions, and will allow distorting the width or height when unchecked.
- Resample image. When checked it allows changing the actual number of pixels in the image.
- The method used is one of the following methods, Basically Bicubic is the best option for most images.
- Nearest Neighbor - Best for Bitmap Images
- Bilinear - Best for Grayscale Images
- Bicubic - Best for Color Images
This is in the "Image" Menu, and it is found by selecting "Canvas Size...".
The purpose of this dialog is to remove or crop portions of the image, making the image smaller by cutting portions of the edges from the image. Width and Height are self explanatory and show the finished size, and there is a pull down to the right to change the unit of measure to Inches, Pixels or Percent among others.
Under this is a graphic that designates which portions of the image are removed to result in the given size. The darkened portion is the image, and it defaults to centered. Clicking on the arrows makes the portion cut to be opposite of the where the selected portion is.