Size is a characteristic of objects in the material dimension. Chakras are not material, they are part of the astral body. Size isn't a thing in the astral in the same way it is here.
So even if we were to have an answer for that, the answer would be nonsensical if we tried to explain it in material terms.
As far as visualization, the Chakras should look relatively small to where they don't clash with each other, but large enough to where details are visible. About fist size.