It would depend on the Blu-ray used. The old ones can hold 50GB on 2 layers. The new ones can hold 66GB on two layers. But, my guess would be the game is much closer to 66GB. Which is probably one reason why there are only 220 premium cars and why a lot of tracks were left out. I assume there are other reasons because Kaz did say that they're releasing the game the want to release, but disc space is probably one of them.
There aren't many games that can fill a Blu-ray. And only 4 titles including GT5 that even use dual layers. However, as I've said, if GT5 is using 33.1GB per layer discs, then GT5 would be the largest game in history.