There were a couple of threads about dummy coils over at Ampage recently. The general concensus was that you should use a coil with no magnet or pole pieces as they dont aid hum cancelling and will only increase the inductance. You want as little load / inductance as posible in the dummy coil as the greater they are the more it will efect the tone of the pickup being humcancelled.
The conclusion seemed to be that what matters is that the turns*area product of the 2 coils match. And as area increases squared in respect to circumference, a larger coil will cancell more hum and put less load on your pickup. Eg..
1" diameter coil, 900 turns, area turns product = 706sq", 2827" of wire.
3" diameter coil, 100 turns, area truns product = 706sq", 942" of wire.
so each coil will cancell the same amount of hum but the 2nd one uses only 1/3 the amount of wire and hence has only 1/3 of the resistance/load of the smaller coil.
thats what i remember anyway, not sure how acurate it is.