I would just loosen them up a bit and push the ones at the bottom to the sides. There can be different approaches .

Sey

I would loosen them, too.

Is this applicable for all plant sizes?

Agree that cutting isn't necessary. You can just break them loose with your hands.

For the most part, yes all plant sizes. The 4' tall one - is that just the plant itself - will have to be trenched in in at least a 3-3.5' long trench. The 6" ones just bury the bottom couple of inches.

Dave

If they are extremely root bound I take a somewhat harsher approach. I rip off the roots at the bottom and rip the rootball open a bit to spread it out and rough up the sides. It will probably shock it a bit but I think it might be better long term. Unless you are very careful with a long bunch of roots they will probably end up going up instead of down anyway. I usually have a bunch of plants so I don't like spending a great deal of time with each one.

Believe it or not water can help loosen up roots. I would just dip in a bucket water and gently loosen the roots that way. That's only needed if it's really root bound though, which is not a good idea to allow happened for seedlings.