Typically landscape rakes (york rakes) are fairly light so lifting shouldnt be a problem on your BX.
They also pull pretty easy since they arent really engaging in the ground like a plow of blade would.
That being said, you would be able to pull even a 8 footer and not have any problems.
The things you have to decide is how manuverable you want it to be. 8' would be sticking out pretty far each side. And if you only have 8' garage doors, it probabally isn't going to fit.
Another consideration is if you get one that is reversable, how narrow it needs to be to be able to reverse while on the 3PH. A 6'er may be close, but a 7' or 8' would definatally need taken off to reverse.
So it is really a matter of personal preference. The larger ones will pull just fine, and get the job done quicker, but at the inconviences I mentioned.