The compendium does have the mount rules. It's in Mounted Combat, under Glossary. It used to be that mounts had to be large or larger, but now they just need to be larger than their rider.
In case you're curious, it is completely legit* for a PC to be a mount, provided the rider is at least one size smaller than the carrier. So you can totally have your gnome warlord ride into battle on a goliath barbarian. But they end up sharing the same set of actions, so it's not really worth it.
*By which I mean, technically legal.