admin-options.php

<?php
add_action( 'admin_menu', 'rfi_admin_add_page' );
function rfi_admin_add_page() {
	add_options_page( 'Require Featured Image Page', 'Req Featured Image', 'manage_options', 'rfi', 'rfi_options_page' );
}

function rfi_options_page() {
?>
	<div class="wrap">
		<h2><?php _e( 'Require Featured Image', 'require-featured-image' ) ?></h2>
		<form action="options.php" method="post">
			<?php settings_fields( 'rfi' ); ?>
			<?php do_settings_sections( 'rfi' ); ?>
			<input name="Submit" type="submit" value="<?php esc_attr_e( 'Save Changes', 'require-featured-image' ); ?>" class="button button-primary" />
		</form>
	</div>
<?php
}

add_action( 'admin_init', 'rfi_admin_init' );
function rfi_admin_init(){
	// Create Settings
	$option_group = 'rfi';
	$option_name = 'rfi_post_types';
	register_setting( $option_group, $option_name );

	$minimum_size_option = 'rfi_minimum_size';
	register_setting( $option_group, $minimum_size_option );

	// Create section of Page
	$settings_section = 'rfi_main';
	$page = 'rfi';
	add_settings_section( $settings_section, __( 'Post Types', 'require-featured-image' ), 'rfi_main_section_text_output', $page );

	// Add fields to that section
	add_settings_field( $option_name, __('Post Types that require featured images ', 'require-featured-image' ), 'rfi_post_types_input_renderer', $page, $settings_section );

	// Minimum Image requirements
	$size_section = 'rfi_size';
	add_settings_section($size_section, __('Image Size', 'require-featured-image'), 'rfi_size_text_output', $page);

	add_settings_field($minimum_size_option, __('Minimum size of the featured images', 'require-featured-image'), 'rfi_size_option_renderer', $page, $size_section);
}

function rfi_main_section_text_output() {
	_e( '<p>You can specify the post type for Require Featured Image to work on. By default it works on Posts only.</p><p>If you\'re not seeing a post type here that you think should be, it probably does not have support for featured images. Only post types that support featured images will appear on this list.</p>', 'require-featured-image' );
}

function rfi_size_text_output(){
	_e('<p>The minimum acceptable size can be set for featured images. This size means that posts with images smaller than the specified dimensions cannot be published. By default the sizes are zero, so any image size will be accepted.</p>','require-featured-image');
}

function rfi_return_post_types_which_support_featured_images() {
	$post_types = get_post_types( array( 'public' => true ), 'objects' );
	foreach ( $post_types as $type => $obj ) {
		if ( post_type_supports( $type, 'thumbnail' ) ) {
			$return[$type] = $obj;
		}
	}
	return $return;
}

function rfi_return_min_dimensions() {
	$minimum_size = get_option('rfi_minimum_size');
	if (isset($minimum_size['width']) && $minimum_size['width'] == 0) {
		$minimum_size['width'] = 0;
	}
	if (isset($minimum_size['height']) && $minimum_size['height'] == 0) {
		$minimum_size['height'] = 0;
	}
	return $minimum_size;
}

function rfi_post_types_input_renderer() {
	$option = rfi_return_post_types();
	$post_types = rfi_return_post_types_which_support_featured_images();

	foreach ( $post_types as $type => $obj ) {
		if ( in_array( $type, $option ) ) {
			echo '<input type="checkbox" name="rfi_post_types[]" value="'.$type.'" checked="checked">'.$obj->label.'<br>';
		} else {
			echo '<input type="checkbox" name="rfi_post_types[]" value="'.$type.'">'.$obj->label.'<br>';
		}
	}
}

function rfi_size_option_renderer(){
	$dimensions = rfi_return_min_dimensions();
	echo '<input type="number" name="rfi_minimum_size[width]", value="'.$dimensions["width"].'"> width (px) <br>';
	echo '<input type="number" name="rfi_minimum_size[height]", value="'.$dimensions["height"].'"> height (px)<br>';
}