I can't stand the "artificial intelligence" built into Word's table feature. You can define how wide the table and/or each column should be in the Table Properties dialog box, but Word then changes the widths according to the text put in the cells.
Word always wants to help as much as it can, and resizing columns automatically to fit their contents must have seemed like a good idea. Unfortunately, AutoFit tends to give people fits automatically.
To prevent Word from resizing the columns, insert a table by using the Insert Table dialog box (see Figure 7-1) rather than by clicking the Insert Table button on the Standard toolbar and "painting" out a table grid. Choose Table » Insert » Table or click the Insert Table button on the Tables and Borders toolbar. Specify the number of columns and rows, select the "Fixed column widths" option, specify a width in the spinner box if you choose, and click the OK button.
If the table is already inserted in your document with AutoFit turned on:
Right-click in the table, choose Table Properties, and click the Table tab if it's not already displayed.
Click the Options button, uncheck the "Automatically resize to fit contents" box, and then click the OK button to close each dialog box (see Figure 7-2).