In planet mode (based on the pics) he will be about 15inches in diameter, with the rings providing the other 15 of the 30 total, so I could see a 20 inch cubed box. In bot mode you're probably not far away with the fort max comparison, but he looks pretty beefy depth wise so who knows how much that would add to the box. Plus all that extra weight, and the stand as well.
It will be a big boi either way. And its 18-24 months away...
We should start a betting service on him - boxed in bot or planet mode? Removeable head? List market odds for potential accessories too (mini Galvatron, mini armada, mini ship, half of a moonbase attachable to his mouth in planet mode etc)