How to prevent resized featured images?

Featured image sizes are mostly likely controlled by your theme, so I would look there. Maybe there is a theme setting for it.

You can also check the following setting to see if it us using one of those sizes.

You can change the featured image size by adding this code into your functions.php file. More info on set_post_thumbnail_size.

set_post_thumbnail_size( 500, 500 );

You can also add a new image size and then use it in your template by doing something like this…

add_image_size( 'featured-image-size', 800, 9999 ); // Add to functions.php
php the_post_thumbnail( 'featured-image-size' ); // Template usage

More info on add_image_size & the_post_thumbnail

Edit: Since you updated your question, here is an updated answer.

You can set the height to be unlimited like this…

add_image_size( 'hitmag-featured', 735, 9999 );