6" will do if you by the right one.
My opinion is by a chipper ASAP, don't get a chuck and duck if you like the skin on your knuckles. 1st year in it was 'Pack and stack', plus stomp and cut and then cut more and stomp. Then I bought a 4" Salsco and changed the game, same truck now holds at least double or possibly triple the material with ease. If you don't buy a vermeer you can get away with not cutting 'y's and just feed the machine, this is what I told a guy who was in the market for one.
"Just so know there is a lot of negatives about the bc 600 or 625, like, for example that the machine is over rated at 6". It is not truly a 6" chipper. It has a 6" tall feed roller and a 6" by 6" opening, meaning it can chip 4" or maybe a 5" branch at the most if you cut it right. It is also designed poorly with a square opening, doesn't take 'y' shaped brush well at all. The newer model the 600XL demoed the new 27 hp kohler which has carb problems for the first year or 2. Also has a really tall chute over 7ft. Makes it hard to chip into a pickup or store in a regular garage. Bandit gives you way more for your money. Is a real 6" machine with a 8" by 12" roller and a 6" by 12" opening with way better engine options. You can rip a 12" spruce log in half and feed the machine and watch the auto feed do it's thing.
Salsco makes a 4" machine with a 7" by 8" roller and a 4" opening that is 8" wide, it is, in my opinion better than the vermeer. Built stronger, designed better with a chute made to chip into a pick up and with a better feed system 2" wider than a vermeer. Takes 'y's better, and is a bigger machine where it counts. Balanced so well take 1 person can pick it up and walk it down the driveway on spin it 180 degrees. There is no way you can do that with the vemeer, Bandits chippers are similar.
Just some friendly advice."
Get some cash go online and find a bandit and go to town, it will make you that money back so fast you won't care about the money it cost you. Just be sure to keep the knives sharp and stay on top of the oil changes, if it has a used gas motor and it doesn't run well, 9 times out of ten it is a carb issue. Diesel's it is all about the way it was broken in and if it has a good cold start. PM me if you need any other advice on starting out.
AshTree