What is it for?
Resizr is for testing the responsive nature of your projects. There are hundreds of different tablet and smart phone screen sizes available these days and unless you work in a large studio you are unlikely to have many of these to hand.
Alternatively it may bit that you are not using the correct protocol at the start of your url, http:// or https://. For best results copy and paste the url from your browser. Would like us to feature your site? You have a nice responsive website and would like it to be featured on Resizr? Drop me a link here.