Fixed height for NivoSlider

I’m using the NivoSlider template and the height of the slideshow changes according to the image height. How I can have it at a fixed height instead of it changing with each slide? I’d use the CoinSlider but I’m not very fond of the transition effects and would simply rather the fade effect which in not included in CoinSlider.

Thank you!

http://wordpress.org/extend/plugins/ml-slider/