......x 100 (feet)
Is the problem, width - too much of it!
I'm learning that a part of my success in learning to control an airplane laterally comes in large part from what you get use to. The runway where I used to keep my 150 was 30 feet wide at the narrow area (after we widened it). When I built my own runway, I went wider - 40 feet between the runway lights, and a bit of clearway beyond that. For the 150, it was lots. When I landed a Twin Otter here a few times, it seemed less. The flying boat I've also owned for a dozen years has about 7 feet from either wing tip float to the runway light, if nicely centered on the runway. So there's lots of incentive to keep the plane lined up on the centerline. And, there's lots of visual cues if you get off the centerline. I usually see that my wheel marks in the grass are laterally within 3 feet one side to the other after a number of takeoffs and landings, so I'm happy with that.
The runway at which I have done the most training of other pilots was actually narrowed fro 150 feet wide (ex military) to 100 feet wide, when it was repaved years back. And the grass crosswind runway (former WW2 training base for Norwegian pilots) is more than 200 feet wide. These immense areas seem to lure pilots into thinking that directional control is not that important. They can wander laterally across the runway, and they're still on pavement. Yeah, they're still on pavement, but they're not as much in control! I opine that narrow runways are better suited to teaching runway centerline discipline!