Image size is pixels (width and height). The image size is set by the camera. You can't change this in Lightroom Classic. You can crop the image and export the cropped image as JPG or TIF, which will give you smaller image size for the exported image, but it will not affect the original image size at all. You can specify smaller pixel dimensions in the exported image, which will give you smaller image size for the exported image, but it will not affect the original image size at all.