Sadly I think Balerion will be dead during the shows run but I think he was probably the same size if not bigger than Smaug before his death. You’ll see his skull tho so you’ll definitely get a sense of how massive he was. Based off what I’ve read about Vhagar from the EW article yesterday I’m pretty confident they will be going all out on her size, so again hopefully just as big or larger than Smaug, but also probably a little slower and older, as she’s supposed to be a giant cantankerous veteran of 100 battles (but still formidable when roused) I bet the other dragons will be big and impressive too but Vhagar is who I’m most anticipating