Well its going to be a pretty damn big ship no-matter what exactly it is. The Pillar of Autumn (ship in Halo 1) was said to be one of the smallest ships in the 'cruiser' class, and even that is over a Km in length.
Assuming those seen in the trailler all come from the SoF it is capable of transporting and deploying at least:
- 8 Scorpian tanks
- 9 warthogs
- 2 unidentified UNSC dropships
- 30 marines
- 5 Spartan II soldiers
- 5 Pelican dropships
- 5 unidentified UNSC figher craft
And obviously a whole lot more than that, considering it was just one battle.