If you look at the top Right hand corner of Task Manager window, you should see, going from Right to Left, a X to close the window, a Square on top of another Square and a little line, like a hyphen.
You can click on the Square-on-a-Square to reduce the size of the window to it's former size. To minimise the window, click on the hyphen-looking icon. If you want to then resize your window, move your cursor to the side or top/bottom of the window and when the cursor changes to double ended arrow, you can click-and-drag the side/top/bottom to the size you want.
Does this help?