You can define them in 2 sections.
1. K2 Categories.
When creating or editing categories look on your right side for lay out grid.
2. K2 Menu items.
When creating or editing menu items linked to categories you can define your settings in basic options. However this only works for multiple categories.
See Joomla Admin > Menus > Main Menu > Create New item > Item type K2 Categories > Select multiple k2 categories and then you will see layout options under Basic Options.
See you around...