The Settings Media Screen controls the various settings related to images and other media that are used in writing posts and pages.
The sizes listed in this section determine the maximum dimensions in pixels to use when adding an image to the Media Library.
This is the size of the thumbnail for any image you upload. Select the Crop thumbnail to exact dimensions checkbox if want the thumbnails cropped to this size instead of scaled.
Setting the medium and large size
When you insert media into a page or post you have the option to set the image size. Small, medium and large. The settings below allow you to define the medium and large sizes (images are scaled not cropped).
This section allows you to choose how to organize your media uploads.
When you have finished making any desired changes click Save Changes.