To quote WPIMGSIZE the default results in a display  pixels wide  pixels if the upright option is used except for those loggedin users who have set a different default in their user preferences In general do not define the size of an image unless there is a good reason to do so some users have small screens or need to configure their systems to display large text forced large thumbnails can leave little width for text making reading difficult In addition forcing a larger image size at say px will actually make it smaller for those with a larger size set as preference