Not much is known about the Forerunner navy, but the little the UNSC has discovered has shown their ships to be very large and incredibly powerful.
ALL FORERUNNER WARSHIPS
Forerunner warships are impossibly large and powerful. Though the current era in the Halo universe has seen very few of them, the ones that have appeared have brought destruction on a massive scale. Without Anodyne Spirit, the entirety of the Covenant likely would not exist. Were it not for the Ur-Didact and Mantle’s Approach, the Raid on Ivanoff Station and the New Phoenix Incident would not have happened. And had the Guardians not existed, the Rise of the Created would not have been nearly as successful.