The used zoom level on a single store page is normally based on the value from the ‘Initial zoom level’ option on the settings page. To overwrite this you have to create a custom store page template and set the zoom level through the wpsl_map shortcode.
So after you’ve created the single-wpsl_stores.php in your active theme folder, look for
[wpsl_map] and change it into
[wpsl_map zoom="your zoom level"]. Any value between 1 ( world view ) and 21 ( zoom in on the street level ) will work.