Set spesific size of featured images

In your functions.php you want to issue a call to set_post_thumbnail_size()

https://codex.wordpress.org/Function_Reference/set_post_thumbnail_size

As with add_image_size, you provide the target width, height & whether you want the image hard cropped to these dimensions. So, for example,

set_post_thumbnail_size(427, 427, true);

will crop all your thumbnails to be a maximum of 427px high or wide, depending on aspect ratio of the original.

If you use

set_post_thumbnail_size(427, 9999);

then your images will be scaled to 427px wide without being cropped.