Register a new image size, then make it selectable in Image Size options

functions.php.php

<?php // Do not copy this line

/**
 * Register a "Wide" image size and add it to the Image Size options
 * after "Large" i.e. Thumnail, Medium, Large, Wide, Full Size
 * to correspond with Align Wide and Align Full alignments
 */

// Add a new "Wide" image size 
add_image_size( 'wide', 1200, 0 );

// Make it selectable in the Image Size options
add_filter( 'image_size_names_choose', 'custom_sizes_choose' );
function custom_sizes_choose( $sizes ) {
	$new_sizes = [];

	// Add new size after 'Large'
	foreach( $sizes as $name => $label ) {
		$new_sizes[ $name ] = $label;
		if ( 'large' == $name ) {
			$new_sizes['wide'] = __( 'Wide', 'textdomain' );
		}
	}
	
	return $new_sizes;
}

Simpler method if you don't care about the order of options: https://gist.github.com/blogjunkie/c220b4f50b06a5d52bb4f2f4cb477908