Windows forms Dockbar shrinking in width when docked

internal class ToolBoxControlForm : DockBar
public static Guid BarId => new Guid(“{e824b645-160f-361d-adf9-c2b457965ac6}”);
public ToolBoxControlForm() : base(Instance.Id, BarId,“”)
var toolBoxForm = new TiaraCAD.ToolBoxControlForm();
//toolBoxForm.Size = new System.Drawing.Size(1000, 1285);
//toolBoxForm.MinimumSize = new System.Drawing.Size(1000,1285);
toolBoxForm.ClientSize = new System.Drawing.Size(1000, 1285);

private void CreateToolBoxDockBar()
var create_options = new DockBarCreateOptions
DockLocation = DockBarDockLocation.Left,
Visible = false,
DockStyle = DockBarDockStyle.Left,
FloatPoint = new System.Drawing.Point(500,500),

m_ToolboxForm = new ToolBoxControlForm();

I am making windows forms dockbar to dock inside rhino
so like suppose in my design view the width of my form is 400to500 but when this RhinoWindows.Controls docks it to left or right side its width decreases
I tried everything like setting minimum size dockstyle but nothing worked
Like i want them to be of width as mentioned in the designer.cs file