1. Open the picture you want to process, and then click "Image" in the menu bar-> "Image size".

2. In the pop-up image size window, set the width and height to the size of A4 paper, that is, 2 10 mm wide and 297 mm high. At the same time, set the resolution to 300 pixels per inch.

Click the "OK" button, and you will find that the picture has been reduced.

4. If you want to enlarge the picture to A4 paper size, you can click "Image" in the menu bar-> "Canvas size" to adjust. Set the width and height to the size of A4 paper, and select the relative option to resize the canvas relative to the size of the picture.

5. finally, you can click file in the menu bar->; Save As saves the picture you are working on.