Set the content width in pixels and adjust the content width for full width pages.

functions-content-width.php

<?php
/**
 * Set the content width in pixels, based on the theme's design and stylesheet.
 *
 * Priority 0 to make it available to lower priority callbacks.
 *
 * @global int $content_width
 */
function bookmark_content_width() {
	$GLOBALS['content_width'] = apply_filters( 'bookmark_content_width', 800 );
}
add_action( 'after_setup_theme', 'bookmark_content_width', 0 );

/**
 * Adjust content_width value for full width page template.
 */
function bookmark_full_width_page_content_width() {
	if ( is_page_template( 'page-templates/template-full-width.php' ) ) {	
		$GLOBALS['content_width'] = apply_filters( 'bookmark_full_width_page_content_width', 1280 );
	}
}
add_action( 'template_redirect', 'bookmark_full_width_page_content_width' );

See list of action hooks: https://codex.wordpress.org/Plugin_API/Action_Reference

Content width for the full width page can't be adjusted before "parse_query" or after "the_post".