Changing the column width

In short, you can't.

Column Width applies to the Column as Row Height applies to the Row.

No you did't. You saw merged cells. That's different.

Merged cells create all kinds of problems. If you want a cell to appear to span several columns, select the cell and the neighbouring cells, then use the Format dialog, Alignment tab, Horizontal alignment and select "Center across selection"

That is a much better way than merging cells.

Change in width is applicable to the whole column but not for selective cells. IT is rightly guessed in above reply that you are confusing merged cells with width.