Wikipedia:Featured picture criteria/Image size

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.

How to determine the size of an uploaded image
Image sizes on Wikipedia can be determined by doing the following: If these numbers do not appear, then the image is smaller than the limit you specified for display in the "Files" pane in your preferences. If you did not adjust that limit, the default size is 800x600 pixels.
 * 1) Go to the image page by clicking on the image thumbnail
 * 2) Under the image there should be a set of numbers in the form "NNNNxMMMM." This is the size of the image in pixels.

Compression
Compression of an image to reduce file size (in Kb) is usually "lossy" and is not advised for featured pictures. Image compression will reduce download times and save disk space, but it does so at the expense of fine detail and overall image quality. If in doubt, when saving JPEG files, always select the "maximum" quality setting. See this page for a fuller explanation and examples of JPEG compression options.

See these rules of thumb for advice on the various image formats.