Crop featured image by default

try this

1. define a custom image size (add_image_size)

   add_image_size(name,w,h,true);

   where w and h for height and width of your image.and fourth parameter true for hard croping of image.read wordpress codex for that

2. put your custom image size name in the_post_thumbnail function

   the_post_thumbnail(‘name’)

or

if your design is not responsive then you can set image size using css.

--class/id-- img{
width:w;
height:h;
}