I like to think that the rift energy allows the Eye to be kept accessible. Remove the rift energy and the power provided by the Eye can't be transferred to the engines. (It's strange that the question of rift energy wasn't addressed in Journey To The Centre Of The TARDIS
Since the Doctor said in The Lodger
that the makeshift TARDIS could destroy the solar system if it exploded, and since he said in Blink
that the Angels could draw enough power from the Doctor's TARDIS to switch off a sun, I think it's reasonable to say that the star from which the Eye Of Harmony was created was at least as massive as the Sun. Since the Sun would apparently have a diameter of somewhat less than four miles if it became a black hole (Exhibit A
, Exhibit B
), it would therefore be reasonable to say that the Eye's diameter would be at least in that ballpark.