for sure there's a thread about this one already?
To ask others for a perfect length of double staff
for you is as asking others for your perfect girlfriend... the answer is only within YOU! (IMHO)
Personally I prefer to have staff (single or double) that I can hold and they still pass my chest with arms extended.. I can do more moves that's all... Just to pass my chest when spun horizontally, then I'm fine can also still do contact and throws. Shorter is harder (for me) to do contact...
But everybody has his own preference, some LOVE (double) staff 6ft long (1,80cm) - with these I would feel limited, I'd always have to watch out whether they hit the floor or myself and I couldn't pick up speed...
So it's about trial and err and can be practised with a regular wodden pole... remember it depends strongly on what you want to do with it...
Good luck