Below is how I'm instantiating the DockPanel (the DeckWorkspace is a valid UserControl... part of CAB, "docked" and "auto-hide" are bools). I have tried setting Size, but that only appears to control the private "dockSize" member, and obviously I don't have access to the private "floatSize" member.
DockPanel panel = new DockPanel();
DeckWorkspace workspace = new DeckWorkspace();
workspace.Name = workspaceName;
workspace.Dock = DockStyle.Fill;
panel.DockState = (docked) ? DockState.Docked : DockState.Floating;
panel.DockState = (autoHide) ? DockState.Docked & DockState.AutoHide : panel.DockState;