The quickest and easiest way is to drag it to the desktop, select it and press "Z" for the preview. Then right click on the preview image and open it in a new tab. Make sure the URL is "original.png" and not "med? or "thumb" or "max". DOwnload that and check the reesolution of the file. Alternative, if you don't mind installing an extension like stylus, I have a style that displays the file size in the list: