You can either add the large size for the generic size through the K2 parameters (you will have to re-upload your images to get that size, though) or you can modify the views you mention above through the K2 overrides in order to achieve this.
to change the image of the user view to Medium. You should make appropriate changes to the other views as well.
If you are not such a code person maybe you should consider asking for help from your template developer if you decide to go with the override modification option.