go into K2 Parameters, and on the images tab, set one of the image sizes to the size you want (probably small or med on your case), then save and close that,
then for just that article: go into the edit item page, and on the right side under "Item view options" go to "Image size" and chose the one you just did above
or if you want all articles within a category to be that size: go into edit category, then on the right side under "Item view options" go to "Image size" and chose the one you you just did above
note your theme may override these settings (like responsive templates sometimes ignore the settings and base the size on screen res.) if your settings are being ignored, test it by changing to one of the built-in templates (like a beez's) to see if it works there
i like to modify things, and make them do unintended things