General: Change Media Crop Sizes
These steps require that you have the Simple Image Sizes plugin installed and active.
After the plugin is active on your site goto Settings -> Media and browse all the media sizes included with your theme.
Using Simple Image Sizes
You can use the new boxes listed in your Settings -> Media screen to customize the built in custom media sizes and crops.
Changes will only effect new images, to recrop all your previously uploaded images, scroll down and click the "Regenerate Thumbnails". You can see what the crop settings are in the table at the end of that screen, "Yes" means hard crop, "No" means soft crop.
You can also change the dimensions of any thumbnail, once again you must use the regenerate button after making any changes to have them applied to already uploaded images.