Oxygen Theme Picture Problems!

Hi! Relative novice to this so if this seems like a stupid question apologies. I am having issues with pictures with this theme. If I post a featured picture in the slider peoples heads get hacked off in the category section which is a thin strip. I have tinkered with this for hours and searched the web for answers but no joy.

What I ideally need is a guideline for dimensions in the media settings section. I have tried 600×200 for the categories section but this is no use for the featured image and vice-a-versa. Any assistance would be greatly appreciated.

https://reversezoom.wordpress.com