To re-size a window means to keep that window in its current position whilst altering its width and/or height. This is done by dragging (moving) the edges or corners of a window into its new width and/or height location. Only a non-maximized (custom sized) window can be re-sized.
As you approach the left or right edge of a window the mouse pointer changes into the Left/Right directional cursor (Fig 1.0) and as you approach the top or bottom edge of a window the mouse pointer changes into the Up/Down directional cursor (Fig 1.1). These indicate that the window can now be re-sized (shrunk or expanded). Likewise, as you approach one of the corners of a window the mouse pointer changes into the Diagonal directional cursor (Fig 1.2) to indicate that the window can be re-sized in any direction (up, down, left, right or diagonally).
Fig 1.0 - Move the cursor Left or Right
Fig 1.1 - Move the cursor Up or Down
Fig 1.2 - Move the cursor in Any Direction
Moving the Up/Down cursor (Fig 1.1) either shrinks or expands the window's height whereas moving the Left/Right cursor (Fig 1.0) either shrinks or expands the window's width. And moving the Diagonal cursor (Fig 1.2) in any direction (left, right, up, down or diagonally) either shrinks or expands the window's width and/or height, depending on how you move the cursor of course (i.e up and then left or up and left together in a diagonal movement). To re-size (shrink/expand) a window just follow these 4 simple steps:
Fig 1.3 - Position the mouse pointer over a window's edge so that it changes into a directional cursor
Fig 1.4 - Click and hold down the left mouse button and then move (drag) the cursor until the window is of a desired size
Fig 1.5 - Release the left mouse button to apply the new window size
The above expanded the window's width and height for the example. However, in reality you might need to shrink two windows to equal size, side by side, so you can compare their contents. For example: You might have two text files open, side by side, so you can compare them and/or copy text from one file to the other. Or you might need to shrink a window so it is not taking up too much space on the desktop but at the same you can still see its contents. These are the normal scenarios.
Take this scenario - You have the DOCUMENTS window (folder) open small. You then create sub-folders and files inside it, but then can not see those new sub-folders and files because the DOCUMENTS window (folder) is not big enough to view all contents (sub-folders and files). In this case you would have two choices. Either re-size (expand) the DOCUMENTS window (folder) or simply maximize it. Remember: Each time you maximize a window you can then restore it back to its original (smaller) custom size. This way you get the best of both worlds - You maximize so you can see all of the window's (folder's) contents and then restore it back to make an uncluttered desktop screen. Shrinking, Expanding and/or Moving a window only can be tedious sometimes, but sometimes that is the only choice you have.
Fig 1.6 - Click on a window's Restore (double squares) button first in order to then re-size it
A window can only be re-sized when it is not maximized (not full size, but custom size) simply because when it is maximized it is using up the whole desktop area (minus the taskbar area) and therefore has no edges. The only way to resize the window is to restore it back to its original custom size first, by clicking on its RESTORE (double squares) button; as above.