Reducing the size of an image, also called downsampling, involves decreasing the number of vertical and horizontal pixels in order to obtain a lower resolution. This takes weight off the filesize, making the image faster to send by email and easier to edit in graphical design software.
You can see the currently open image's size in the upper-left hand corner just above it; its dimensions are stated in pixels. Listed next is the number of megapixels, i.e. the surface area taken up by the image. One megapixel is equal to 1 million pixels, which would be the area of an image 1,000 pixels long and 1,000 pixels wide, for example. Note how pixels are calculated differently than bytes, with one megabyte equal to 1,048,576 bytes.
The most straightforward way to shrink an image is to reduce its initial number of pixels by a factor of two, three, or four. The respective downsampled files would be one-half, one-third, and one-fourth their original dimensions, and take up 4, 9, and 16 times less disk space.
If the original file size is not a simple multiple of the new, downsampled size, Photo Lab will automatically perform pixel averaging to preserve image quality.
An image's size should be reduced in a way that avoids the need to make it larger again later, as any pixels lost along the way cannot be recovered.