From experience I have found the the chips/shreds from my Cub Cadet CS.3310, will grow grass in about a year. I don't know what the particle size is, but the literature for the chipper/shredder says a 10:1 reduction. Ground up branches comes out about the size you would expect, if you cut the tree down with an axe.

About a year ago we had a problem with our septic system and the flower bed near the tank became a morass, not realizing what was going on I placed my wood chips in the area. About 6 month later I planted over the wood chips and about 6 month later, (this spring) I have nice grass growing in the area.

I use the Cub Cadet to take care of the branches and brush that comes off of my lot. Recently I ground up enough material to produce about 6 bushel of chips.