1. Photoshop: Image size and resolution

2. We work with PPI

6. Changing image size to view on a display in Adobe Photoshop
You may need to reduce the size of an image to send a photo to your friends or upload it to a web-site. To change the image size in Adobe Photoshop to view it on the screen, you should:
Step 1. Select the command Image - Image Size from the Adobe Photoshop menu.

7. Step 2. Activate the check-box Constrain Proportions to keep the ratio of the image dimensions.
Step 3. Make sure that the check-box Resample Image is activated.
Step 4. In the Pixel Dimensions group select the unit measure and enter new values for the parameters Width and Height