Set the default Featured Image (formerly Post Thumbnail) dimensions. To register additional image sizes for Featured Images use: add_image_size().
<?php set_post_thumbnail_size( $width, $height, $crop ); ?>
To be used in the current theme's functions.php file.
add_theme_support( 'post-thumbnails' ); set_post_thumbnail_size( 150, 150 );
This function will not resize your existing feature images. To set net size to existing images use regenerate thumbnail plugins
Set the default Post Thumbnail size by resizing the image proportionally (that is, without distorting it):
set_post_thumbnail_size( 50, 50 ); // 50 pixels wide by 50 pixels tall, resize mode
Set the default Post Thumbnail size by cropping the image (either from the sides, or from the top and bottom):
set_post_thumbnail_size( 50, 50, true ); // 50 pixels wide by 50 pixels tall, crop mode
set_post_thumbnail_size() is located in