i would imagine that the ships wont be exactly scale as some forerunner ships were on the same scale as the plot device supercarrier. But i would also imagine that the average forerunner capital ship would be the same size as the assault carrier, if not bigger
HAHAHAHA!
Forerunner fortress class- 100km (Human/Forerunner war- 50km varient)
Covenant plot device Super Carrier- 27km
The super carrier isnt even on the same scale but u are correct though, it would be advised we dont have the sins engine howling in agony by hurling several game domain breaching game models, safe to say all the 7km plus ships will have to be taken in a little,
hell even ^this^ is a narrow view of the Forerunners build scale especially when u get into the planet and enclosed planetary orbit construction sizes