How to get post thumbnail to be equal to set media size

the_post_thumbnail allows for size specificiations. So you would do:

<?php the_post_thumbnail('medium'); ?>

if you changed the image dimensions in the admin, you will need to go back and re-upload the images so they crop to the correct size that you set up

If the issue is the dimensions of the image you are probably looking for a hard-crop. The best bet is to set up a new image size with add_image_size in your functions.php.

add_image_size( 'new-image-name', 250, 150, true );

then re-uploading images then

<?php the_post_thumbnail('new-image-name'); ?>