From what I've been told, you want 5 PTO horsepower for every foot of implement.
So, if you have 30 PTO hp, then a 6' implement will be fine. However, I've also seen that, depending on the implement and environmental conditions, you can adjust that ratio down (say for a finish mower on grass you can go like 4 PTO hp per foot) ... or you should raise it up (say you're using a tiller in very hard soil, or brush hogging down some crazy thick stuff - then you might want to to 6-7 PTO hp per foot).
That's what I've been told anyhow...