i use a split button, which has a RadContextMenu attached to it. Inside the RadContextMenu, there is one RadMenuItem which contains an ItemTemplate with multiple RadCheckBoxes.
How is it possible that the RadCheckBoxes inside the ContentTemplate uses the full available width? So there should be no padding/margin in the ContextMenu and the Checkbox stretches horizontally to use all of the existing space. I uploaded an example of how it looks like right now in the attachments of this post.