How to reproduce: just create a RadForm, in the designer change its size and set its FormBorderStyle property to be None. When the form loads its size will be increased.

Workaround: set its MaximumSize property
this.MaximumSize = new Size(400, 80);

Please refer to the attached screenshot and sample project.

Workaround: call the RadForm.EndInit method before showing the form.

Workaround: set the RadRibbonFormBehavior1.AllowTheming property of the form to false

public class RadForm1
{
	public RadForm1()
	{
		InitializeComponent();

		this.RadRibbonFormBehavior1.AllowTheming = false;

	}
}