Changing physical size of image may require changing size or resolution.
Resolution – number of pixels per inch square.
If resolution of image does not match resolution of device displaying it, image resolution must change.
If image does not fit destination space in current resolution then image needs to be resized.
Downsampling – keeping size image size, reducing number of pixels – pixels look bigger.
Upsampling – keeping size, having bigger image attained by adding more pixels.
Upsampling and downsampling known as resampling.
In Photoshop, Image > Image Size.