There is no setting to make the tables autofit the pauge. However, you could use a macro like the following and assign it to a keyboard shortcut, then run it on the table you've just inserted:
Code:
Sub FitTable()
With Selection
If .Information(wdWithInTable) = True Then
With .Tables(1)
.AllowAutoFit = False
.PreferredWidthType = wdPreferredWidthPercent
.PreferredWidth = 100
End With
End If
End With
End Sub
This is more flexible than using a fixed width, as it allow the table to resize to fit whatever margins & page layout you're using (such as when changing between portrait and landscape).