Hi
Start by understanding that K2 settings are
inherit or
overruled.
So if you go to K2 Component > Parameters (on the top right side) you can set with in px for default images sizes. Those are the settings that the component will use for all, except if you add different size on some category (this will overrule the component settings).
Even if you do this step you can even change size on specific k2 item that will overrule the component and category settings.
Hope you understand the way it works