I can’t see the thumbnail with it’s proper regulation. By default, the thumbnail shown 200X300, 600X1200, 600X1024. If i use 600X1200 or 600X1024 it’s not show in site properly. But if I use 200X300 then show properly but the resulation is so bad.
I want to see my thumbnail size as 600X400. But how?
I would accept that you already have added this image sizes in your functions.php. In order to get the proper image output, you need to regenerate your thumbnails to set the proper thumbnail sizes. Download and install Regenerate Thumbnails and regenerate all you thumbnails. You only need to do this once.