Getting something sized 2-3x the pool size makes sense. Anything much more than 3x could have issues with setting the on-time percentage low enough, especially if your pool isn't in full sun or you get a lot of cloudy days in the spring/fall. You can certainly check the specific model to see if they let you dial down to 1%, but if they only have 10% increments, then that might not be good enough, especially if you run your pump for a long time (say, with a 2-speed or variable speed pump that runs at a low speed but for a long time).