The scale of the porch and the structure the columns support will dictate their size.

How big is the room?

Keep in mind that you will still need a handrail up the stairs that might look funny if it doesn't die into a guardrail up top. Maybe just keep the handrail and guardrails to the left of the stairs to define an area that is more intimate than the rest of the porch? R311.7.8 Handrails. Handrails shall be provided on at least one side of each continuous run of treads or flight with four or more risers. Also, note that the 30" drop is not measured straight down from the edge of the porch. The slope of the landscaping for drainage away from the house usually takes up some of that 30 inches, if you are that close to the limit. R312.1.1 Where required. Guards shall be located along open-sided walking surfaces, including stairs, ramps and landings, that are located more than 30 inches (762 mm) measured vertically to the floor or grade below at any point within 36 inches (914 mm) horizontally to the edge of the open side. Insect screening shall not be considered as a guard. Also, if you are still under the 30" limit everywhere, after taking the ground slope into consideration, realize that you do not need to comply with neither the minimum rail height nor opening limitations, which can open up a whole new list of possibilities, aesthetically, and yet still provide some degree of minimal protection from someone stepping off the porch when not paying attention. For instance, something like a bench can often be a good buffer, or a shorter, wider rail that works well for setting a beverage on while sitting out there in a rocking chair. R312.1.2 Height. Required guards at open-sided walking surfaces, including stairs, porches, balconies or landings, shall be not less than 36 inches (914 mm) high measured vertically above the adjacent walking surface, adjacent fixed seating or the line connecting the leading edges of the treads. R312.1.3 Opening limitations. Required guards shall not have openings from the walking surface to the required guard height which allow passage of a sphere 4 inches (102 mm) in diameter.

Can anybody please tell me what size square columns I should be putting on my ranch style home? 8 inch or 6 inch?