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
Click and drag the boundary left or right to change the width
Double-click the boundary to AutoFit the column 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 and adjust one boundary to change all selected columns together
Use AutoFit Column Width from the Home tab, then Format
Drag the boundary between column headers to resize manually