A raider's motto is the Eleventh Commandment, "Thou shallt not get caught."
If you make a unit that has four or less CP's, then it is classified as a small unit. You can check this yourself. Make a cavalry Division with just four cav. elements. Click on the Division and cursor over the icon in the bottom right corner of the screen that looks like a dot with a ring around it. You get a message that tells you the evasion value of the unit. Add a fifth cav element and you'll see that the evasion number goes way down. The way evasion works, the program adds up all the patrol values of the opposing unit trying to find you and then divides this total by your unit's evasion number. The higher your unit's evasion number, the smaller their patrol's chance of finding you. Terrain and weather affect this too. If they can't find you, then they can't kill you.
Partisans, rangers and small cavalry units make great raiders. Cav. and rangers are very mobile and the partisans can use an RGD card to blow up depots. Any opposing force unit in a region blocks supply progress through that region. If you want to twist up RR tracks, the success rate depends on the strength of the unit divided by 100. So a four element cav Division with a strength over 100 will automatically succeed. This unit also has a fair chance of chasing off the auto-garrison of a rear area depot. You could even have several 4-element cav Divisions move through enemy lines and then concentrate into one Division at the point of attack in a rear area to close off a supply or retreat path. Rangers are very fast and can get you a view of what is going on deep in your enemy's backfield. Just remember that all raiders will need to get back to their own supplies eventually.
Additionally, a small unit leader cannot use the evade combat command if he is inactive, so a good strat number helps. Some RGD cards, like the Cavalry Screen, make your raiders harder to find. Be aware that other RGDs can make them easier to find, too. You should only raid when it is a critical target and the mission is quick in, quick out.
http://www.ageod.net/agewiki/Pillage_and_Raid
I'm the 51st shade of gray. Eat, pray, Charge!