Dragging column edge with the mouse to set width manually:
See also Row Heights
Double clicking column edge to set width automatically:
Using the context menu (right click) to set the width manually:
Using the ribbon menu to set the width manually:
Setting the width of multiple columns at the same time:
Selecting non-adjacent columns, rows and cells all work in the same manner. Hold the Ctrl key down and click on each of the items you want to select.