Resizing changes the pixel dimensions of an image. The difficult part is not typing a width and height; it is deciding what should happen when the new shape does not match the original.
Aspect ratio explains the shape
An aspect ratio compares width with height. A 1920×1080 image is 16:9. A 1080×1920 image is 9:16. A 1080×1080 image is 1:1. Two images can have very different pixel counts and still share the same ratio.
Preserve, crop or stretch?
Preserving aspect ratio changes the size while keeping the original shape. Cropping removes some content so the image can fit a new shape. Stretching forces the source into the requested width and height and can distort circles, faces, logos and text.
For most photographs, preserve the ratio unless the destination requires a specific shape. When a specific ratio is required, crop intentionally rather than stretching.
Downscaling versus enlarging
Reducing dimensions throws away pixels, but it often still looks good because the final display is smaller. Enlarging asks the software to invent intermediate pixels. It can make the file larger, but it cannot create genuine detail that was absent from the source.
Common dimensions are not universal requirements
1080×1080 is a useful square format, 1080×1920 is a common vertical canvas, and 1920×1080 is a common landscape canvas. Always check the destination because websites and apps can change their recommended sizes.
File size comes afterward
A resized image may still exceed a KB or MB limit. Once the dimensions are correct, use compression to reduce the final file weight.