How to get the code string for the gMaps function in page editor

This is very simple to do.
Go to maps.google.com and paste in the address to the maps search bar and when the map appears you'll see in the top left of the page a link (for paste link into email or IM) and then paste that address into the gmap generator in the page editor.

The Google Maps script will then insert the relevant shortcode with the Google Map reference into the shortcode.

You can set the size of your gMap by going to wp-admin/settings/Google-Maps/ and insert your preferred map size in there.