I am trying to find a way of 'turning off' the titles and descriptions (and other items overlaying the image) if the size of a responsive slider becomes below a certain height or width. The converse is also true - is it possible to stop the images/slider getting any larger over a certain width?

For the next couple of weeks see nzlodge.org for why this is asked for especially when you reduce it to phone size

Hello,
Using media queries you could just se display:none for the .nivo-caption class.

As for the size - nivo slider fills 100% of the container it is placed into. So just set a max width of your wrapping container.