Step 1
Look at the top right-hand corner of your browser window. Notice three little symbols, there. Concentrate on the middle one.
The middle one should resemble either a single square or a couple of overlapping squares.
If it resembles a couple of overlapping squares, then click it so that it changes to a single square.
If it resembles a single square, you are ready to proceed.
Step 2
Place your cursor at the right-side margin or at the left-side margin of your browser's window.
(Your cursor should change shape when you have positioned it at the margin of the window:
it usually changes from a pointer to a double-sided arrow.)
Push the window border inwards.
Pull the window border outwards.
Find the size you like.
Restore
If you want to restore the full screen, then just click the little single square (upper right-hand corner of your browser's window).