Laying out the 12 temples around spawn. Each temple gets a chunk. It's position in the circle determines the element.
I thought some temples will be in the Nether and some in the End.
The problem with scattered temples in various dimensions is people will rarely see them or visit them. Especially with twelve of them. By having them around spawn, they're more likely to be seen. Another option is we could call these the Altars of Element and people can build scattered temples of they want.
If we go with altars, the 1x1 chunk size is fine. I also added an image of where I would put 3x3 chunk size spaces if we want to do temples around the spawn, or if we don't think temples can fit in a 1x1 space. Keep in mind that the marked space is just a maximum, builders can do whatever they want within the marked space. They don't have to cover every block, and they can change anything they want within that space.