Here is the Image Size for Raise Mag 

• Image Size for Grid/List Large Thumbnail 800 X 500 px 
• Image Size for Grid/List Small Thumbnail 707 X 442 px
• Image Size for Grid/List Large Thumbnail 800 X 500 px
• Image Size for Author Thumbnail  800 X 600 px
• Image Size for Grid/List Large Thumbnail 800 X 500 px
• Slider Three Thumbnail( Fullwidth ) 1311 X 600 
• Slider Four Thumbnail 500 X 700 
• Widget Post Thumbnail 300 X 300 px

But If you maintain Image SIze 1311 X 600  then it will automatically crop the image as of its size.  Minimum your image size must be 1311 X 600  or 800 X 600 px. 

Note: If you’ve switched the theme, don’t forget to regenerate all the thumbnails. Otherwise, images will be displayed in undesirable sizes. Use Regenerate Thumbnails, plugin to regenerate thumbnails.