Go to Image in the menu bar and select Image Size. You can also hit Alt + Ctrl + I (Option + Command + I) on the keyboard.
In the box that pops up, be sure to check the box that says Resample. This ensures that Photoshop will create more pixels to add to the image.
There are a few different options for how Photoshop will perform this feat. Most of the time Automatic or Preserve Details work great. However, some of the others may work better with certain images so feel free to play around.
If you want the aspect ratio (height and width in relation to one another) to stay the same, make sure that the chain-link button is checked. Uncheck to freely resize horizontally or vertically.
You can also choose different measurements to use from the dropdown box that says pixels in my example. We’ll just stick with pixels for now.
Now, notice the quality of the bird in the preview panel above. Let’s bump it up to 6000 pixels wide, keeping the same aspect ratio, and see what we get.
My image is still zoomed in to 100% and you can see the quality is beginning to degrade. But…we just tripled the size of the image. The results might not be perfect, but they are quite impressive.
You can also change the size by playing with the resolution. For example, here I changed the resolution to 150. Instead of 300 pixels per inch, the program will convert the image to 150 PPI.