Used self contained might be a thing to look at , 6 " is the minimum useful size, with 8" being about the most popular, followed by the 10 inch. How much are you going to be useing it? Lot of Brush clearing? If a lot of Brush you definitely want something with a good sized intake that is self feeding. takes forever to trim stuff down smaller to fit some of those tiny intakes. My current unit is a Echo Bearcat 5" 20hp chipper shredder it fits my needs- but it does take time to trim all the branches off , the intake is 5x5 manual for the chipper portion the shredder will handle up to 1" dia. items, about 18" sq. intake hopper on that portion. I do not have to jam any thing unless the blades get exceedingly dull- I never let them get that far. I have run 5" diameter buck-thorn dry and green through it and never had to help it or hold the the trunk back because it was over loading and bogging the drive. Gravity fed, once the blades get into it feeds itself. Chuck and ducks name comes from horizontal manual feed until the blades catch the material- have a habit of spitting back out the intake hence the duck part.