Slider – Is there an optimal size?

Hi there,

Just recently started using this theme (loving it so far!) but have a question about the Slider images… is there a particular size that is best used for this feature?

I ask because when some images which are quite large and I just assumed it would mask the parts of the image outside of the supported Slider size, it instead squishes them down to an odd level. Other images it seems to mask properly (or at least how I would expect it would) and others it displays perfectly fine. I can’t quite find a perfect size that works consistently, nor can I find any documentation on the recommended sizes/resolutions.

Can somebody please explain their findings or point me in the right direction of some proper documentation? Thank you in advance! ??