Did you make sure that you have a dynamic form and not static?
Select the comments fields andmake sure that allow multiple lines is set in the object tab. Then go to the layout tab. Make sure that the height is set to expand to fit (this is a checkbox).
You also have to make sure that the subform / page that this control is on has a flow layout, so that it expands to accomodate your dynamic comments.
Sorry for the duplicate message. - After a refresh I didn't see Srini's comments which are the same as mine.