Function Reference/set post thumbnail size

Description
Set the default Featured Image (formerly Post Thumbnail) dimensions. To register additional image sizes for Featured Images use: add_image_size().

Note: To enable featured images, the current theme must include add_theme_support( 'post-thumbnails' ); in its functions.php file. See also Post Thumbnails.

Usage

 <?php set_post_thumbnail_size( $width, $height, $crop ); ?>

Parameters
$width
(int) (optional) The post thumbnail width in pixels.
Default: 0
$height
(int) (optional) The post thumbnail height in pixels.
Default: 0
$crop
(boolean) (optional) Crop the image or not. False - Soft proportional crop mode ; True - Hard crop mode.
Default: false
Examples
Default Usage
To be used in the current theme's functions.php file.

if ( function_exists( 'add_theme_support' ) ) {
add_theme_support( 'post-thumbnails' );
        set_post_thumbnail_size( 150, 150 );
}
Crop Mode
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
Change Log
Since 2.9
Source File
set_post_thumbnail_size() is located in wp-includes/media.php.

Related
Post Thumbnails: has_post_thumbnail(), the_post_thumbnail(), get_post_thumbnail_id(), get_the_post_thumbnail(), add_image_size(), set_post_thumbnail_size()