There isn’t a set size to go by – as background images respond to size of the browser. But if you create a wide image say 1600px x 200px then this should fill the space on your site nicely when viewed on the average size screen
For the fonts to display in the Customizer you will need to add this PHP snippet to your site:
add_filter( 'generate_typography_default_fonts', function( $fonts ) {
$fonts[] = 'Open Sans';
return $fonts;
} );
Change Open Sans
to the name of your font.
This article explains how to add PHP:
https://docs.generatepress.com/article/adding-php/