Revolution Slider Resize in Pro

Hi, I am using the revolution slider like so:

https://onedrive.live.com/redir?resid=5060D4EDA2F65A67!289&authkey=!APKqrwiLo-_V8ew&v=3&ithint=photo%2cpng

As you can see the image is showing in full on the screen.

However, when I resize the browser window slightly so that it is a little thinner, he image stays deep but crops the edges like so:

https://onedrive.live.com/redir?resid=5060D4EDA2F65A67!288&authkey=!AAWRYeXY7dxHFhg&v=3&ithint=photo%2cpng

I want it to resize in proportion, so that the width of the image is maintained (and not cropped). I’m happy for it to be shallower and not so deep so that this is possible.

How do I set it to do this?

Many thanks