try this
-
define a custom image size (add_image_size)
add_image_size(name,w,h,true);
where
w
andh
for height and width of your image.and fourth parameter true for hard croping of image.read wordpress codex for that -
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;
}