Like Tropical Jack above, we have a
Woodmaxx 8"
chipper and in my case 30+ PTO HP. But it's close enough to provide some advice to you at 20 PTO HP. Any hydraulic infeed
chipper that will let you control or reverse the infeed speed will let you chip anything your HP will allow. My experience is a bit limited so far but I can certainly say that even with 30hp at the PTO I wouldn't want to be fighting with a gravity feed
chipper where there's no way to change your mind after you feed it something that will stall the tractor. With infeed control, you can:
slow it down to chip larger stuff...
feed it in spurts if it's just too huge to chip even at slow speeds...
stop it from feeding if the tractor is stalling or for any other reason...
or just give up and reverse it out of the chute and not chip it.
20hp at the PTO should be more than capable of some very decent chipping, just no way you are going to have constant feed above around the 4" diameter range. BUT, I'd still want at LEAST a 6" infeed if not 8" simply because even at 20 PTO HP you can still chip entire 1-2" trees and brush without limbing it. It's not about the biggest thing you can chip, rather what can you chip steadily successfully at slow speed and can you just shove the whole thing at it without extra work. Slower is potentially ok... extra work isn't. Stalling the tractor constantly and burning the
chipper belts and putting extra wear on the tractor PTO will just be a miserable and costly fight.
Personal preference here but no matter the HP I wouldn't want to bother chipping with only a 4" opening because of ease of use. 6" minimum 8" preferred even at 20hp. We have just over 30 at the PTO with an 8"
WoodMaxx MX-8800 and frankly it's just awesome. But it would really, really NOT be awesome if I couldn't adjust the infeed speed as needed... or just put an entire small scrub tree in and walk away.
Years ago I had one of those small engine personal gravity feed home size branch and leaf chippers and it made me think chipping was one of the dumbest things ever. Now on our larger wooded property this monster 8" with horizontal infeed destroys 6 foot high 30 foot long piles in 1-2 hours of not working too hard.
The moral of the story is, don't underbuy the features on a
chipper if you actually intend to use it. This isn't one of those times where you will get away with it. If a
chipper doesn't really work well and easily, you won't use it. Just burn or haul it away instead.
Two of the most popular options today for compact tractors that will work at 20 PTO HP:
Woodland Mills sells hydraulic infeed chippers for a reasonable price. The WC68 is a Chinese made 6" model and has full speed control and reverse on the infeed. 20hp minimum.
WoodMaxx sells both Chinese and American made hydraulic infeed chippers priced accordingly to where they are made. The WM-8H is the Chinese made 8" model and has full speed control and reverse on the infeed. 200lb flywheel and 19hp minimum.
Either of these appear to be a LOT less money than the average price of the Land Pride WC1504. Which is a bit surprising.