Featured Image size and caption

Hi there, I’d like to change the size of the featured image on the home page (make it smaller, as I feel it’s too big for my purposes). I also would like to include image caption underneath the featured image (centred) to give credits to the creators of the image. I went into index php and tried to change 300,300 here:

<?php
if( wp_is_mobile() ) { $postimg = ‘full’; } else { $postimg = ‘medium’; }
echo dez_get_featured_post_image(“<div class=’post-thumb in-archive’>”.$thepostlink, “</div>”, 300, 300, “alignleft img-is-“. $postimg, $postimg, dez_get_singular_cat(‘false’), the_title_attribute(‘echo=0’), false);
?>

that didn’t make any difference.

I included line

<?php echo get_post(get_post_thumbnail_id())->post_content; ?>

This added the caption line, but in the wrong place – I don’t know how to move it under the image and centre it there.

Any suggestions would be much appreciated.

Many thanks!