Limit search to custom field

$type="bananas";
$args=array(
    'post_type' => 'bananas',
    'post_status' => 'publish',
    'posts_per_page' => -1,
    'orderby' => 'meta_value',
    'order' => 'ASC',
    'meta_key' => '_bananas_size',
    'meta_query' => array(
        array(
            'key' => '_bananas_size',
            'value' => $_GET['size'],
            'compare' => '>=',
            'type' => 'NUMERIC'
        )
    )
);

$my_query = null;
$my_query = new WP_Query($args);

You can read more about the parameters for the WP_Query here. Also, some type of data validation and sanitization is recommended.