There is one option you can try: You can assign an absolute size to the border dock zones.
The DockPanel consists of five dock zones: left, right, top, bottom and the document area (center). The size of the left, right, top and bottom dock zones can be defined using the properties DockLeftPortion, DockRightPortion, DockTopPortion, DockBottomPortion of the DockPanel.
If these values are <1 then the value is treated as a proportional size relative to the DockPanel. The default value is 0.25 which means that the dock zone will take up 25% of the DockPanel - unless it is resized by the user. The dock zone will automatically resize when the parent window is resized.
A value >1 is treated as an absolute size in pixel. For example, if you set
dockPanel.DockRightPortion = 200;
then the right dock zone will have a fixed width of 200 pixels. The right dock zone will not be resized when the parent window changes. (But it can still be resized by the user.)
But the space inside one of the dock zone is always proportionally divided among all docked forms. (It is not supported to explicitly constrain the size of one particular form.)