I don't know that I have ever seen one under 30' and some up to several hundred feet in diameter. In farm ground they still hold water after a rain and having been farmed over for 50 plus years.
Admittedly, the ones I’ve seen were on relatively small pastures.
Costs a lot of money to try to fence in a large area to contain bison.
Any way we look at it, bison are hard on the neighborhood. Doesn’t mean that they’re not tasty, just means that they’re not exactly a meek “environmentally friendly” animal.
However, I would love to see a herd of bison big enough that we could try to induce a bunch of environmentalists to try to milk some bison females. That, I would pay to watch. ;-)