I'd start by checking my operator's manual. It should give you the maximum size and weight of tiller for your tractor. From there go shopping. Some are smaller and heavier and others are bigger and lighter. Find the one that best suits your tractor, ie., covers the wheel tracks of your tractor. There's nothing more time consuming than having to offset the tiller, till a row and have to run all the way back to the start to till the next row. The price range on them is quite large depending on how well built they are and the manufacture's name. I have a Kuhn EL 32 which is just over 4 feet. I run it behind the ol' B2400. It's one of those implements that I ran the heck out of when I first got it but these days, not so much, maybe an hour a year.