具有大小到视口但也具有最大宽度的Fancybox

I use Fancybox2 (fancybox.net) on my website. When the ligthbox is shown I want it to size according to the screen width of the browser, so it's good to view on both mobile and desktop. However, on the desktop I want the box to have a maximum width of 500px.I use Fancybox2 (fancybox.net) on my website. W