How to get 300x300 image url for the search result grid?

you can use the below hooks code

// The filter callback function.
function example_callback($data)
{
    foreach ($data['allPosts'] as $key => $value) {
        $thumbnail_url = wp_get_attachment_image_src(get_post_thumbnail_id($value['ID']), array('300', '300'), true);
        $data['allPosts'][$key]['medium_thumb'] = $thumbnail_url[0];
    }
    return $data;
}
add_filter('reactivepro_get_grid_all_data', 'example_callback', 10);

Then in your grid template code replace the img src with below code

<img src={{post.medium_thumb}} alt="medium thumb image"/>