Image size really makes a difference in the look of your documents. The size of your document and its resolution are determined by where your image is going to be viewed.
This includes magazines, flyers, posters, and anything else that is printed. The higher the resolution, the better the quality will be of the image. The standard resolution is 300ppi, or pixels per inch. It can go even higher, for posters and large banners, but keep in mind that the higher the resolution is, the larger the file size will be.
Web images are viewed on a screen, so the resolution can be lower. Most images look good at around 72ppi, but since many monitors are now larger, some are starting to use 90ppi. For consistent and expected results, use 72 ppi, as it is the industry standard.
A major part of image size is the quality. You can take a large image and resize it to make it smaller, and you will not lose any quality, but you can’t take a small image and blow it up to be clear and crisp in print. It doesn’t matter what resolution it is, if you take a 2 inch by 2 inch image that is 300ppi and resize it to be 6 inches by 6 inches, you will lose quality, and it may even be unusable.
In Photoshop, you control the size of your document in two ways. The first one is via creating a new document, where you can control the width, height, and resolution. The top drop down menu also has presets, such as standard web sizes, sizes for mobiles devices, sizes for film and video, and print sizes, which automatically sets the width, height, and resolution to the industry standards that you find every day.
The other way is to go to Image in the menu at the top of the screen of your open document, and select Image Size. here, you can take an open document and resize its width, height, and resolution. This enables you to take high resolution print images and save them at a lower resolution for web and mobile devices. This also enables you to see the document size versus pixel dimensions. In the example we see that the current size of the document is approximately 2.25MB, and the width is 1024 pixels, the height is 768 pixels. the actual physical size of the image is 14.222 inches by 10.667 inches.
It is always a good idea to to have constrain proportions checked in the options at the bottom, because it automatically adjusts the width proportionally when you change the height and automatically adjusts the the height when you change the width, which keeps your image in proportion, and keeps it from appearing to be stretched.