For a textbox in a report, you can use the CanGrow/CanShrink property. In Report Design View, with the textbox selected, goto Properties - Format and set them to yes.
While you can find these properties in Form Design View, they don't work on forms, another stellar move by the Microsoft developers! To do it in a form, you'll need to use Stephen Lebans CanGrow hack for forms. You can download it here: