Resize a Window
Resizing a window on your Mac is very much like resizing a Windows window. Move your mouse pointer around so the tip of your pointer is on each edge and corner of the window. Notice how the mouse pointer changes to a double ended arrow indicating something can be changed. This means when you see that little changed pointer, you can (while holding down the left mouse button) drag the size of your window in or out, up or down to make it larger or smaller. If you have the mouse pointer on the edge of any corner, you will see it become a diagonal double ended pointer. This means you can (left click and hold) drag the window out or in to make it larger or smaller two ways at the same time.
It takes practice to get the mouse to do what you want. Resizing windows and moving them around is a great way to get that practice without worry. You can't hurt a thing!