This is how I make forms full screen.
private void button1_Click(object sender, EventArgs e)
{
int minx, miny, maxx, maxy;
inx = miny = int.MaxValue;
maxx = maxy = int.MinValue;
foreach (Screen screen in Screen.AllScreens)
{
var bounds = screen.Bounds;
minx = Math.Min(minx, bounds.X);
miny = Math.Min(miny, bounds.Y);
maxx = Math.Max(maxx, bounds.Right);
maxy = Math.Max(maxy, bounds.Bottom);
}
Form3 fs = new Form3();
fs.Activate();
Rectangle tempRect = new Rectangle(1, 0, maxx, maxy);
this.DesktopBounds = tempRect;
}