Well, here's my $.02. 3/4" is pretty much useless except for keychain/AAA lights. 1" will work for AA & 123/17670 lights. 1.25" is good for 18650 lights and lights with 25-30mm reflectors. C & D Maglite heads are a whole different story. Get the smallest diameter that will do the job. The less you have to turn down, the less waste, and waste is money. If you can fit the desired size through your headstock, get the longest lengths you can afford. This will cut down alot of waste also, as you won't be left with a useless stub in the chuck every time you make a part.
Jeff