Select the column or columns you want to adjust
Move the cursor to the right edge of the column header until it becomes a double-headed arrow
Drag the edge left or right to set the width manually
Double-click the right edge of the column header to auto-fit the width to the contents
Right-click the selected column header and choose Column Width
Enter the desired width and click OK
Use the Home tab, then Format, then Column Width
Select multiple columns to apply the same width to all of them
Drag the boundary between two column headers to resize one column
Use AutoFit Column Width from the Format menu to fit the widest cell content