At the most you can use a 5' box scraper but a 4' is the perfect size for the L1801. It does take some practice to getting really good with a box scraper. A 6' box scraper is just way to big and will having spin your tires in short order in my experience of using box scrapers. I got a 4' for my L1801 and it did a dandy job. You will be happiest with the 4 footer as it will overlap the rear tires by about a inch on each side.
To use the box scraper you need to keep adjusting the upper link to tilt the scraper back slightly and if that ends up to aggressive a half turn of the upper link and then keep trying until it pulls the material you are moving to get the desired effect.
To use the box scraper you need to keep adjusting the upper link to tilt the scraper back slightly and if that ends up to aggressive a half turn of the upper link and then keep trying until it pulls the material you are moving to get the desired effect.