To reproduce :
public Form1()
{
    InitializeComponent(); 
    
    this.radDropDownList1.DropDownListElement.DropDownHeight = 400;
    this.radDropDownList1.DropDownListElement.DropDownWidth  = 400;
}

Workaround:
this.radDropDownList1.DropDownMinSize = new Size(400, 400);