To reproduce: 
1. Add RadForm and set the following properties at design time: 
this.AutoSize = true;
this.WindowState = FormWindowState.Maximized;
this.Size = new Size(750, 500); 
2. Add RadMenu and add more than 10 menu items
3. Run the application and you will see that menu items are overlapped (see the attached image)

Workaround: 
Restore the WindowState of form to Normal at design time and set the WindowState to Maximized run time in the Load event of form. Here is the snippet: 
private void RadForm1_Load(object sender, EventArgs e)
{
    this.WindowState = FormWindowState.Maximized;
}