Source

File: wp-admin/includes/media.php

function image_size_input_fields( $post, $check = '' ) {
	/**
	 * Filters the names and labels of the default image sizes.
	 *
	 * @since 3.3.0
	 *
	 * @param string[] $size_names Array of image size labels keyed by their name. Default values
	 *                             include 'Thumbnail', 'Medium', 'Large', and 'Full Size'.
	 */
	$size_names = apply_filters(
		'image_size_names_choose',
		array(
			'thumbnail' => __( 'Thumbnail' ),
			'medium'    => __( 'Medium' ),
			'large'     => __( 'Large' ),
			'full'      => __( 'Full Size' ),
		)
	);

	if ( empty( $check ) ) {
		$check = get_user_setting( 'imgsize', 'medium' );
	}

	$out = array();

	foreach ( $size_names as $size => $label ) {
		$downsize = image_downsize( $post->ID, $size );
		$checked  = '';

		// Is this size selectable?
		$enabled = ( $downsize[3] || 'full' === $size );
		$css_id  = "image-size-{$size}-{$post->ID}";

		// If this size is the default but that's not available, don't select it.
		if ( $size == $check ) {
			if ( $enabled ) {
				$checked = " checked='checked'";
			} else {
				$check = '';
			}
		} elseif ( ! $check && $enabled && 'thumbnail' !== $size ) {
			/*
			 * If $check is not enabled, default to the first available size
			 * that's bigger than a thumbnail.
			 */
			$check   = $size;
			$checked = " checked='checked'";
		}

		$html = "<div class='image-size-item'><input type='radio' " . disabled( $enabled, false, false ) . "name='attachments[$post->ID][image-size]' id='{$css_id}' value='{$size}'$checked />";

		$html .= "<label for='{$css_id}'>$label</label>";

		// Only show the dimensions if that choice is available.
		if ( $enabled ) {
			$html .= " <label for='{$css_id}' class='help'>" . sprintf( '(%d&nbsp;&times;&nbsp;%d)', $downsize[1], $downsize[2] ) . '</label>';
		}
		$html .= '</div>';

		$out[] = $html;
	}

	return array(
		'label' => __( 'Size' ),
		'input' => 'html',
		'html'  => join( "\n", $out ),
	);
}