If you want to re-size the window
while keeping the maximized state, then you're doing it wrong.
But I'm assuming rather that you want to re-size it
from a maximized state. In this case, you just have to do as joshduncan2012 suggested, and change the windowstate back to normal.
this.WindowState = FormWindowState.Normal;
this.Size = new Size(yourWidth,yourHeight);