You'll probably do OK with either a 5ft or a 6ft, but I'd suggest the 5 foot size. I've found that the horsepower demand is not as much as one might expect. Your IH 300 should have enough power, even if it is a bit tired, but it will also depend on your soil conditions. Heavy, wet clay will demand more power than light sandy soil, for example, and breaking new ground is harder than tilling ground that has been worked previously. Rocks, roots, etc. will be a problem - be sure you get a tiller with a slip clutch.
You might find the 6 footer a bit heavy on the back, requiring some front weights to keep the front wheels down, particularly if you are going up hill.
You can get 60" models that can be offset so they cover one of your wheel tracks. Unless you have a lot of ground to work, the 5 footer will work just as well as the 6 footer, and they are cheaper as well.