First you need to setup the width of all the sizes that an image is resized to when uploaded in K2. You can do this globally (K2 parameters) or even per category (category parameters). Make sure one of these sizes (e.g. M for Medium size) is 400px wide. This will restrain your images horizontally to 400px. Provided your users won't upload portrait images (tall), you can enforce the height of 200px using some CSS. E.g.
.catItemImageBlock a {display:block;width:400px;height:200px;overflow:hidden;}
.catItemImageBlock a img {display:block;}
Alternatively, change the template by creating an override inside your Joomla template (read this
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates) and then place the image of the item as background image in the image container and use background-size:cover to make the image stretch to fill your 400x200 px container. More on that here:
developer.mozilla.org/en-US/docs/Web/CSS/background-size 
