I have a discussion running on this matter if it can help anybody.....

community.getk2.org/forum/topics/k2-add-new-item-window-resize?xg_source=activity

This is to resize the box, but as per this thread, K2 are ignoring request to incoporate it, any new updates (l2.4/2.1) overwrite these changes and you have to do them again, but I hope this helps,

Jasper