You can configure Media Settings as you like. Go to Settings > Media Settings.
Placeholder Image: In this section, you can set a media placeholder image.
Large Thumb Image Size: In this section, you can set the large thumb image width and height.
Medium Thumb Image Size: In this section, you can set medium thumb image width and height.
Small Thumb Image Size: In this section, you can set the small thumb image width and height.
