How effective are medium and large drones against smaller targets?
Medium and large drones cause significantly more damage than small drones. But they are also much slower and have much worse tracking.
The Vexor can field a theoretical maximum damage set of drones consisting of 2x Ogre II, 2x Hammerhead II and 1x Hobgoblin II. While this gives the theoretical maximum damage, I'm wondering if the Ogre II can actually apply significant damage on cruiser-sized targets. If they could not, then using all medium drones might be a better idea for cruiser fights.
I'm ignoring flight time for the moment, as this depends a lot on the distance of the drones and is something I can calculate with EFT/Pyfa. I'm mostly interested in the effect of tracking on attacking smaller drones, and I can't see a way to simulate that with either EFT or Pyfa.
What are the minimum target sizes for medium and small drones above which they can apply most of their damage effectively? How effective would large drones be against AB-fitted cruisers?
Although Task's answer is good, like with most things in EVE online, situation dictates. For the purposes of this discussion we will use Gallente drones (you have a Vexor after all).
Hobgoblin max speed: 2,800
Hammerhead max speed: 1,680
Ogre: 840
Obviously the MWD bonus of the Algos and the Dragoon will impact this, but you get the idea.
Most PVP frigates worth their salt will be going at least 2500, and most beyond even that. In this case the likelihood of any drones hitting is pretty nil. With the re-balances, even most cruisers are going well above Ogre speeds.
However, many combat Vexors fit single and double web, and a scram to shut off micro-warp drives, dragging the targets speed down substantially. If you can manage to catch your enemy and drag them down to < 400 m/s then Hammers or Ogres could still be effective.
For PVE I have found the biggest issue is travel time to raise your isk/hour and to reduce the tedium. In that case I generally pack lights or mediums. My Rattlesnake brings long and close range sentries, and a flight of lights to handle frigates.
I have found in practice that tracking doesn't factor in nearly as much as simply catching up. If the target is slow enough for the drones to out run, they can probably hit. Ogre's are medium sized weapons and has 2x the tracking of Electron blasters on a Vexor 0.36 vs 0.185. So basically tracking is not the issue.
Example: http://zkillboard.com/detail/27695426/