Right, here's a few things
(pulls up Halo: Genesis from my CE Halo Wars)
It appears to be the Covenant have Omni shields and are heavily laser based. Combined nuclear weaponry firepower can take them down, but one covenant ship needed 13 other ships to die before it was defeated.
There is also a HUGE discrepancy between the power of covenant and UNSC ships.
107 UNSC vs 13 Covenant under Admiral Preston Cole, end result is victory at the loss of 37 warships.
Seventy left, 8 more covenant, ONI dubbed it a strategic victory despite the loss of 30 more ships.
Final battle, 'Battle of the Great Bear', seventeen ships (at least eleven destroyers) against three covenant, all three covenant killed with one crash landing with the loss of said eleven destroyers.
UNSC Spirit of Fire, the converted colony ship of Halo Wars fame, has a repair and refit bay capable of launching exoskeleton vehicles and up to 60 EVA engineers. MAC cannons accelerate depleted uranium slugs to a fraction of the speed of light, Archer missile pods, storage/auxiliary launch houses for scorpion tanks, warthogs, self assembling fusion reactors and other items that can be dropped planetside via pelican assist.
Unshielded covenant ships of small size die within one shot of the SoFs MAC. Havok nuclear missiles too.
They seem to be fairly large, ranging upwards of a few hundred metres (this is the Prophecy, BTW)
They do have stealth ships like the UNSC Last Gleaming.
Hope this helped...
