The recommended featured image sizes are those set through the theme options:
– height is directly configurable;
– width is computed based on site width and posts layout;
Images are normally resized and cropped to size, so if the originals have a very different aspect ratio from the set values, then more cropping will occur.
Cropping position can be adjusted or disabled entirely (with the possible side effect of blurry or distorted images, depending on how numbers match up).