.nivoSlider { height: 300px; }
If you are doing it responsively, you need to figure out the correct width and height dimensions according to the media query being used. Below are just random numbers so you can see what I mean:
/* Smaller than standard 960 (devices and browsers) */
@media only screen and (max-width: 959px) {
.nivoSlider { height: 300px; }
.nivoSlider img { height: 300px; }
}
/* Tablet Portrait size to standard 960 (devices and browsers) */
@media only screen and (min-width: 768px) and (max-width: 959px) {
.nivoSlider { width:300px; height: 200px; }
.nivoSlider img { width:300px; height: 200px; }
}
/* All Mobile Sizes (devices and browser) */
@media only screen and (max-width: 767px) {
.nivoSlider { width:200px; height: 100px; }
.nivoSlider img { width:200px; height: 100px; }
}
/* Mobile Landscape Size to Tablet Portrait (devices and browsers) */
@media only screen and (min-width: 480px) and (max-width: 767px) {
and more here
}
/* Mobile Portrait Size to Mobile Landscape Size (devices and browsers) */
@media only screen and (max-width: 479px) {
and more here
}