"Pixel Dimensions", "Document Size" and "Resolution".
General information about resizing images for web and print use, and what's the difference between "Pixel Dimensions", "Document Size" and "Resolution" can be found on these websites.
Resizing an image using Adobe Photoshop.
With your image open in Photoshop, click "Image" from the menu bar at the top of the screen.
From the choices that drop down, click "Image Size".
A dialog box like the one pictured below should appear.
First, make sure the 3 check boxes at the bottom are ticked - "Scale Styles", "Constrain Proportions" and "Resample Image".
In the "Pixel Dimensions" section at the top, enter a number in either the "Width" or "Height" box.
These two boxes are linked so theres no need to enter a number in both. The other one will look after itself.
The values in the "Document Size" section - "Width", "Height" and "Resolution" are important if you intend to print your image.
They have no relevance if the intended use is a website or for digital projection.
Click OK
Your image should be resized and ready to save.
To save your resized image, click File from the menu bar at the top of the screen.
A list of choices like the one pictured below should drop down.
Click "Save As". You will then be prompted for a location to save your image.
Choose a location, give your image a File Name and make sure the Format is JPEG.
Click "Save".
As you have chosen JPEG for the format, you have a choice of how much to compress the file size of your image.
A dialog box like the one pictured below will appear and it is here where you can specify the compression rate.
The compression rate is expressed in terms of quality. The more compression, the smaller the file size and the lower the level of quality.
Drag the slider between "small file" and "large file", taking note of the number on the right ( 85.5 K in the picture ).
Aim for as large a file as possible within the constraints of the use of your image.
For instance, images for competitions must usually stay under 500 K.
Click OK.
Your image is now in the location you indicated, saved as a JPEG and sized accordingly.
See the "Competition entry" page for more information on image sizes and file sizes for images you wish to enter into competitions.