You can manually set the column width by hovering over the join between column headers. (The cursor will change to an East-West arrow), click and drag.

AutoFit Selection will size the selected column(s) to the widest entry in it.  This can be done without going through this menu option by simply highlighting the column(s) and hovering between any of these column headers, (you will get a East-West arrow cursor) and double-clicking.

Hide will hide selected column(s). This is useful if the column(s) do not need to be displayed but may have sub-calculations in them. Remember you can hide and unhide column(s) in 'View>Custom views'.

Unhide will unhide column(s) that have been previously hidden. You will either need to select the column(s) both sides of the hidden one(s) or use 'Edit>Go to...' and select an address in the hidden column(s) and then take this option.

Standard Width... allows you to set your own default standard width for the current workbook, via the dialog shown. This is handy if, while building a worksheet, you are having to widen or shrink every column. Excel reverts to it's installed default when opening a new workbook.