Any size of debris field can be harvested, even those that do not pass the minimum size threshold to appear on the galaxy view page.
When the recycler arrives at a debris field, it fills its holds evenly with all resources until the lowest resource runs out, then fills the rest of the capacity with the remaining resource. For example if a debris field had 40,000 metal and 15,000 crystal, a single recycler (20k resources max) would harvest 10,000 metal and 10,000 crystal with 30,000 metal and 5,000 crystal remaining in the debris field. For the same debris field, two recyclers (40k resources max) would harvest 25,000 metal and 15,000 crystal, with 15,000 metal remaining in the debris field.
Note that any ships sent along with recyclers on a harvest mission are inactive participants. If one recycler and 100 cargo ships are sent to a massive debris field, the maximum resources that can be harvested is still 20,000. The cargo capacity of other ships is not counted towards the harvesting capacity of a harvest mission.
Though the debris field is said to orbit a planet, the defenses of that planet do not protect the debris field, and neither do any ships in orbit of the planet. The same is true of moons in orbit around the planet: they do not defend the debris field with their defenses or orbiting ships. This means that debris fields are free and available, completely undefended, to the first ships that can reach them. The player who owns the planet from which the debris is being harvested has no way of knowing that ships are incoming on a harvest mission, although they will know the debris is harvested if they watch the galaxy view page and notice the debris field is gone.
The dangers facing a fleet on a harvest mission do not occur at the debris field, but at the planet or moon to which it will eventually return. If a harvest mission is sent from a planet, the entire mission (including arrival/return times) will be visible on any nearby phalanx, and an enemy can time their attack to land seconds after your harvest mission lands. If you launch a harvest mission from a moon you are much safer since your mission can not be phalanxed, however enemies can (and do) keep an eye out for disappearing debris fields and attempt to calculate a harvest mission's return time accordingly in an attempt to land an attack fleet at the originating moon seconds after the harvest mission lands (see blind phalanx).
A harvest mission can be recalled at any time before it actually reaches the debris field. Once it has harvested the debris, it behaves like a recalled mission, and it can no longer be modified. This is why harvest missions should never be used to fleetsave, since any enemy can determine your fleet's return time with a phalanx, and you will not be able to recall or otherwise affect your ships' return times.
How is it done? Edit
There are several times in OGame in which you would want to harvest resources in a debris field.
After a battle Edit
If you have sent ships and you have won (or lost!) a battle, there will likely be debris in orbit afterwards. After the first few weeks, some players find there is more profit to be made in crashing an enemy fleet than simply raiding for the resources on the planet.
The best tactic to use when harvesting after a battle you have initiated is to send a single Espionage Probe on an attack mission to the target planet. This espionage probe will lose the fight, and create a 300 crystal big debris field. This will give you a debris field to send your Recyclers to.
Recyclers are much slower than other ships in OGame, with the Death Star being the only ship that has less speed. Because of this, if you send an attack mission and it lands in an hour, but your recyclers don't get there for 4 hours, it means someone else, closer, can steal the resources which you have worked for. So as outlined above, send the recyclers first to the tiny debris field you've created. Look at how long your attack-fleet will take and time it's launch so it will hit the planet about 5 minutes before the recyclers land for the 300 crystal in orbit. Assuming you've sent enough recyclers, they will take all the debris - 300 crystal plus the amount generated by the fleet battle.
Random searching Edit
You will see debris fields on the Galaxy View which can be identified by the "Red Space Dust" picture next to the planet name. If you hover your mouse over the "Red Space Dust", you will see how much Metal and/or Crystal is there (denoted by Metal: 900 for a debris field of 900 Metal or Crystal: 900 for 900 Crystal).
If you have Commander you can click on "Harvest" link from a planet which has recyclers in its fleet and it will launch the appropriate amount of ships automatically!
If not, look at the debris field, work out how many recyclers you need to send ($ \left ( Metal + Crystal \right ) / 20000 $ ) and then send them on a harvest mission (if the answer is a decimal, round up so; 5.3 recyclers = 6 recyclers). Be sure to select "DF" from the drop down menu next to the target co-ordinates so you get the Harvest Mission option for the mission-type.