When you unpin a pane, we follow what VS does which is initialize the size of the flyout based on the size of the pane (unless the root split pane it exists within hasn't been resized in which case the preferred - i.e. desired - size of the pane is used). Once the user resizes the flyout that size will be used until the pane is repinned at which point when it is unpinned again the next time, a new size will be calculated using the same logic mentioned here. So with regards to changing the size, if the flyout has not been resized and the root split pane the pane was in was not resized at the time the pane was unpinned, you can just set the Width of your content pane. For the other cases, currently you cannot control the flyout size. You may want to submit a suggestion for exposing a method for changing the flyout size.