Hello,
We bought a Bosch ATX 2200 garden shredder years ago which claims to handle up to 40mms 40mm. It uses a toothed gear against an aluminium cutting block and is very slow and prone to not cutting through the wood properly. Ate miles of hedging but not up to the job now we're cutting back willow and pine, branches around 3" - anything bigger makes firewood.
Given the poor performance of some 6.5hp petrol chippers on youtube I'll give them a miss and look for something PTO driven, or stockpile the branches and hire a FORST in the summer.