i am in the tree service industry. there are 2 common types of portable chippers, on is the old asplundh ''chuck and duck'' ''wisper chipper'' or drum chipper, that rips stuff out of your hand and the disk chipper, that autofeeds stuff in slowly after you release it. the disk chipper gives a much more consistant chip size. almost any tree service will gladly bring you a load of chips from local tree work for no charge.if they use a bandit or vermeer brand chipper, it will probably be a disk chipper. the drum chipper tends to shread or strip things, so long pieces are possible. the disk chipper has 1 or more blades on a large heavy flywheel spinning aroundand the size stays about 1/4 inch thick by 2/3 inches across. but there are variations as well as fines even from this process. you could make a screen tumbling trommel, though, to seperate the sizes, if you had to. like a barell made out of chicken wire, to let the fines fall out.