"The Spirit of Fire" ship would most probably be a carrier based cruiser. Something like a hibrid ship. Why-
Here are the two ships lined up:
UNSC Battlecruiser- Basic Review:
This ship has massive armor, intirely made of Titamium-A material. Has no less than 20 "Archer" Missile Silos over the whole body of the ship. Each silo contains 10 missiles. Has upto 3 MAC guns, varying by size from small to super (depending on size of the cruiser). Also has gun turrets all over the body of the ship used for anti-air defence and operated by the ship's AI. Can carry a small "LongSword" Fighter squadron and a "Pelican" boarding vessel squadron. Also contains the quarters of a small army of marines and all the training area needed. Can make a Hyperspace Jump.
UNSC Supercarryer- Basic Review:
This ship is a purposebuilt carryer with armor intirely made of Titanium-A. Lightly weaponed, with only one MAC gun. Also has more than 20 "Archer " Missile Silos, with 10 missiles in each silo. Has gun turrets over it's body for anti-air defence operated by the ships AI. Carryes 20 "LongSword" squadrons, 18 "ShortSword" Bomber squadrons and a significant "Pelican" boarding vessel squadron. Contains upto 4 Platoons of "Hell's Airborne"-ers and 4 Marine Platoons with all the training area needed, including enough ammonitions and fuel to last them 6 months of intense fighting. Can make a Hyperspace jump.
So if "The Spirit of Fire" is an extended version of the Supercarryer, purposly built for the job of exterminating the Rebells and then the Covenant Bugs, it would be one sweet ride. Of course it could be fust a carryer, a Supercarryer that is....
"Exitus Acta Probat"
"The result justifies the deed"