If displaying latest K2 items from a menu all images are set to medium size by default. See helpers/utilities.php line 246 that attempts to get parameter to set image sizes in K2 latest items view mode. I can't find this parameter in the CON_K2 table or k2_categories or k2_items.
Result is all images in latest k2 items view are unsizeable.
See here for category view with small size images set
blackdog.co/spring/blog/es
and here for latest items view with small size images set
blackdog.co/spring/blog
Cheers
Dave