Storms tend to get bigger as they last longer and intensify. With major hurricanes, eyewall replacement cycles tend to help bring that about by reconcentrating the maximum heating with the storm outward. With weaker storms, it's largely through environmental interactions. The further north a storm gets, the larger it is likely to be, also likely a function of environmental interactions. Otherwise, there aren't many set rules on storm size; features that start out smaller (larger) tend to stay smaller (larger), but that's about it.