For the purposes of examining featured pictures, image size normally refers to the pixel dimensions of an image (often referred to as resolution) and not its size in kilobytes (kB). Size in kB varies with image content and JPEG compression and generally bears little relation to pixel dimensions. In general, it's a good idea to crop a photograph to highlight the relevant subject. Bear in mind that, after cropping, an image must be a minimum of 1500 pixels along its shortest edge to be considered for featured picture promotion.