following codes under your mainForm_Load method -
private void mainForm_Load(object sender, System.EventArgs e)
{
//setup screen size first
int scrHeight = System.Windows.Forms.SystemInformation.PrimaryMonitorSize.Height;
int scrWidth = System.Windows.Forms.SystemInformation.PrimaryMonitorSize.Width;
this.Size = new Size(scrWidth , scrHeight);//this sets
}