Problem with resizing to large screens

Hi. I am working on the website

https://www.commonthreaddesign.co.uk

My client was extremely particular about her design, and so I had to create my own using Artisteer, so it’s not a standard WordPress theme.

The issue is that it displays fine on my screen, and a normal 19″ monitor, but on the larger screens it looks very small. Does anyone have any suggestion as to how I could get around this?

Thanks