Any bale can vary in weight for the same size, depending on how it was made.
The only way is to buy by the tonne or fraction of a tonne - however unless you are buying from a lorry that's been weighed, you can still only guess !
We work on the basis of 40 small bales to the tonne, ie., 25kgs each. However, we make good solid bales and a lot that are sold to horsey folk will weigh a lot less than that

Large round bales are generally between 3 and 5 to the tonne.
Large hestons vary, can be as heavy as 2 to the tonne, or only half that.
Mini-hestons vary even more, but most are probably around 8 to the tonne, ie., about 5 small bales'worth each.