I searched online, all answers are like below:
"To set a column to a fixed width, do one of the following:
Click the column header menu, and then select Make Column Fixed Width.
Make Column Fixed Width specifies a width for the column (in the code) that matches the current visual width of the column.
Select a cell in the column by clicking an edge of the cell, then click Fixed and type a numeric value in the Property inspector.
If you enter a numeric value that is less than the width of the column's content, Dreamweaver automatically sets the width to match the width of the content."
I can't find the column header menu. Please help!