a detailed design will follow soon enough, for now this is all I can give you guys a sneak peek of..
But surely Rassilon would've had a new Tardis built for him for the time war based on his old design but with state of the art time lord technology... and why not with offensive weaponry? It would be his 'flagship' after all.
Also considering Rassilon's ego, I think it would be fitting for him to enable his Flag-Tardis to materialise in its true expanded form for intimidation purposes. (you may disagree)