You should first create a child theme of Sugar & Spice. Otherwise, any changes you make will be lost.
Once you’ve created a child theme, copy the function sugarspice_setup() from the parent theme’s function.php file into your child theme’s function.php file. You want to begin with the line that reads:
if ( ! function_exists( 'sugarspice_setup' ) ) :
and go down to (and include) the line that reads:
add_action( 'after_setup_theme', 'sugarspice_setup' );
Then in your child theme’s copy of the setup function, look for the line that looks like this:
set_post_thumbnail_size( 210, 210, true );
and comment it out by doing this:
/* set_post_thumbnail_size( 210, 210, true ); */
This should give you full size thumbnails.