i got the same difficulty with the custom image size.
I dont get it in fact.
if i set some css for the suroundiv div or on the custom-slider css class, i loose the responsive capacity of the slider, right ?
when i use the function add_image_size() as suggested, nothing happened, i mean the slider use a unknown (from me) size and not mine. (i removed the slide, the featured image before setting the custom size as you said earlier)
So, how do i set the slider to use my image size, let's say 966*425 for example please ?
thanks a lot for this plugin ! looking forward for the next release