28" sounds about right for a 28' boat. Any smaller and you won't have the leverage necessary to steer easily and it will be a long reach if you are sitting on seats or coaming.
One downside of a wheel, is getting around it in the cockpit. At the dock when we have guests, we'll take the wheel off as we will do when working on the boat.
It is tempting to take the wheel off when anchored, however, if the anchor drags or the boat needs to be moved quickly, getting the wheel back on will add to the confusion of moving the boat.