The image size is set in K2 Properties as the Global Settings.
Image settings can also be set in Categories and Items.
If your Items follow the settings of a category, then the settings would be entered there.
You can also set an item to not follow the category settings, and set individual items accordingly.
If no settings are applied to the category or to the item, it should follow the settings in the Global Settings (Properties) for K2.
It is hard for me to tell what is going on without seeing all the settings.
Check your settings and your origianal image size.
You are not showing us an image examples of why you think this is not working. Because it is a Global setting, does not mean that it will complete that setting by entering a number in the field. You will only see the result when you actually create an item with an image that follow the rules you have set up.