To quote WP:IMGSIZE: "the default results in a display 220 pixels wide (170 pixels if the "upright" option is used), except for those logged-in 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 260px will actually make it smaller for those with a larger size set as preference.