lightbox size

Hi,
I’m using “Highend Image Frame” element with “open lightbox” on click action. How can I set this lightbox to be 90% (width or height) in relation to the browser window?

Now, on click it opens the image in a relatively small lightbox which is useless, and if I click the expand button in the top right corner it opens the image in his actual size without scaling to the browser window which is useless again.

Thanks,
John