First question, is it a 3 point attach or a trailer type, either can be PTO. If its 3 point, you may need considerable counter weight on the opposite side to offset the leverage of the sickle bar. My dad had about a 42" on a Sears garden tractor in the mid 60's, had over a hundred pounds of wheel weights on the left rear wheel to offset the bar on the right, this was mid-mount. unit, and you still had to watch out for tipping. I have used a JD 2355 with a 7 footer on a 3 point, and you could feel a lighter left side. Now if a trailer type, it should work with anysize tractor as long as it has the PTO power as it does not depend upon the tractor for stability, and if its a wheel drive unit PTO is not even a consideration.
Hope this helps. My advise, look for something in the 4 to 5 ' lenght for smaller tractor.