I finally solved this =)))
Turned off Auto-height slider, set Minimum height amount to 0 (zero) and used slides fillmode Stretch. In the CSS-file for my iPhone-settings, I wrote (tune to the max-height that works for you):
@media only screen and (min-device-width:320px) and (max-device-width:480px) {
div.master-slider-parent { max-height:200px }
div.master-slider { max-height:200px }
div.master-slider img { max-height:200px }
}