A 3" aero is generally plenty for a single 8" woofer. A 4" is also going to require a lot of length; you'll have to custom make it with your own PVC in order to get the length required. .8ft^3 at 40hz with a 4"er requires 24.5" of port length.
A 3" will work well on low to mid level 10" subs, and a single 4" will work well with the same low to mid level 12" subs. The flares really help eliminate port noise. If you're just wanting as much output as possible, then feel free to go with the larger port.