I got the solution so self-answering the question for future users/visitors.
Hope it will help someone.
I think this won’t be able to generate results based on the Post ID →
the_post_thumbnail( 'large');
However, this should work →
<?php echo get_the_post_thumbnail($fpid, 'large'); ?>
(Above is succesfully tested)
Other possibilities →
<?php echo get_the_post_thumbnail($fpid, 'thumbnail'); ?>
<?php echo get_the_post_thumbnail($fpid, 'medium'); ?>
<?php echo get_the_post_thumbnail($fpid, 'full'); ?>
If you want to use the hardcoded POST ID then this should work →
<?php echo get_the_post_thumbnail('311', 'large'); ?>
Other possibilities(Hard Coded POST ID) →
<?php echo get_the_post_thumbnail('311', 'thumbnail'); ?>
<?php echo get_the_post_thumbnail('311', 'medium'); ?>
<?php echo get_the_post_thumbnail('311', 'full'); ?>
P.S. → 311 is a post ID, for example.
$fpid → User selected POST ID(Post selected form the Theme customizer
settings)
I hope this should be helpful to someone in future.