If the browser "window" is not "maximized" you can probably
see some of your previous screen around it. You will also notice
that in the window's upper right corner there are three boxes
containing a dash, a square, and an X -- like the graphic at the right.


If you place the mouse cursor over the square with the box,
a caption that says "Maximize" appears.(Try it on the graphic.)
Clicking on the square will make the window take up the maximum amount of screen space allowed. This is what you want.

If the window is already maximized, the center square will contain
two overlapping boxes, like the graphic to the right. Placing the
mouse cursor over it will bring up a caption that reads "Restore."
(Try it on the graphic.) Clicking on the square would return the
window to its smaller size. Don't bother.