Your mind set on this is actually wrong, though I say that respectfully. It is not the SIZE of the stump, but rather WHAT SPECIES of tree the stump is. This is in terms of removal and in rotting.
For instance, I have great big Eastern Hemlock here, but I can push them over with a modest sized bulldozer because those types of trees tend to have a root ball closer to the top of the soil. In contrast, White
Ash have a top root that goes down to hades where Lucifer is grabbing a hold of it and will never let it go. I am being silly here, but the point is, I would rather push a 2 foot Eastern hemlock stump then a 10 inch White Ash stump. The same is true of White Pine...darn soft tree, but suffering goodness pulling those stumps is like a new dentist pulling a lower back tooth! But Oak...for at is seeming strength...they pop right out, and
apple trees have such shallow roots that an ATV could push their stumps out of the ground. But rot factor is another matter. A White Ash is anchored to the ground for sure, but they rot incredibly quick. That is not the case with easy to pull, but rot resistant White Oak. So, it really is not about size of the trees.
I clear a lot of land, for myself as well as others, though the last thing I wanted to get into for
retirement was land clearing. There are many ways to do it, and in the farm classes I teach, I discuss many of them, and will here upon request. Mostly though it has to do with time. The more time a person is willing to wait, the less money they spend. The best way is to wait 5 years after logging the wood off before even attempting to clear the stumps. That is because the stump rots from the ground up, and not the ground down. If a person clears right after logging they get a much bigger stump. This takes bigger equipment, that burns more fuel, and costs more to rent/buy/maintain. However, today few people want to wait. they want trees cleared today, and fields tomorrow. It can be done, and it still is worthwhile to do so from an economical standpoint, but more expensive.
Another issue however, is what to do with the rootball. Here in Maine I can still burn them, which is against the law in Vermont, though I have no idea about the southern states. Even then it is a liability because they burn forever. I am talking a year or so and they are still smouldering, but greatly reduces teh size of them. A farmer gets fertilizer out of teh burned stump though (0/1/3) and
biochar. Pushing them into a ravine is always nice, or to the edge of the field; all depending on the length of push. That is what makes production. When a person is spending half their time backing up with a bulldozer, it is easy to see where that would affect production.
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Many people have tried using livestock to clear land, and I suppose it is possible, but I have yet to see anyone who have tried this, not to give up halfway through and bring in the equipment. There is nothing wrong with that because as I said, time only helps landclearing, even if equipment is used.
For what it is worth, Land Clearing for me costs about $201 per acre, but there are a lot of variable in that. To have it done here costs about $3000 per acre.