How to make lightbox bigger on mobile and tablet

Hi, I am really happy with the galleries I have created on my site but the lightbox that opens when you click on an image on mobile or tablet is too small and I want to make it larger. Can you tell me how I can do this please? I want it both wider and taller but still be responive. You can view the lightbox here

http://www.melaniemay.com/portfolios/photographs-connemarra-ireland/