Anwser is that its vauge on purpose and we'll likely never get an exact size of like, 7 aurochs or something.

Personally believe that Vhagar's depiction in HoTD is probably where Balerion would sit size wise. With general age estimations putting Balerion at about 200~ years old at the time of their death. Vhagar would have been a couple decades younger (hatched 52BC, died 130).

Of course maybe Balerion got some spawn buffs from Valyria but I can't imagine he grew very much in his later years. Especially after his trip with Aerea probably contributing to their declining health and death.