I'm not familiar with your tractor. But I used a 72" landscape rake on a 30hp Kubota B2910 and I kept looking behind me to make sure it was there. 72" is an easy pull for a 30hp tractor. I would say that 84" to 96" could be pulled by a 30hp. My NH TC24D is probably very well suited to the 72" rake with its 24hp engine.
I would actually say that you could safely pull 1.5 times the width of your tractor.
Unless you have any narrow areas, I'd get the biggest one.
More to do the job faster, and if the 8ft. is angled, it'll still be nearly 6', covering tire tracks.
edit: make sure you can still turn the rake around and not hit the tractor. Taking the rake off every time one wanted to change position would get annoying pretty fast.