A preparatory note before we begin. John Gillingham refers to the style of warfare practiced in high medieval England and France as Vegetian. The name comes from a late Roman military writer who was enormously influential on medieval Europe. The essence of Vegetian warfare is that large pitched battles are risky and best avoided; if you lose, you lose your army, and may not be able to raise another. A combination of skirmish, raid and siege was the preferred method of waging war. Raiding weakened the opponent and made it easier to besiege his castles and strongholds.

In both Richard I and "Richard I and the Science of War," John Gillingham includes a translation of a passage from the Chanson des Lorrains which viscerally describes a marauding army:

The march begins. Out in front are the scouts and incendiaries. After them come the foragers whose job it is to collect the spoils and carry them in the great baggage train. Soon all is in tumult. The peasants, having just come out to the fields, turn back, uttering loud cries. The shepherds gather their flocks and drive them towards the neighbouring woods in the hope of saving them. The incendiaries set the villages on fire and the foragers visit and sack them. The terrified inhabitants are either burned or led away with their hands tied to be held for ransom. Everywhere bells ring the alarm; a surge of fear sweeps over the countryside. Wherever you look you can see helmets glinting in the sun, pennons waving in the breeze, the whole plain covered with horsemen. Money, cattle, mules and sheep are all seized. The smoke billows and spreads, flames crackle. Peasants and shepherds scatter in all directions.

We can unpack this a bit. First, we see that the goal of the marauding army is twofold: one, to "forage" - that is, to steal - and two, to destroy that which they are incapable of carrying off. Foraging was how you paid for your war and kept the army supplied; ravaging embarrassed and weakened your opponent socially and economically. As to "how do you burn down an entire settlement?" We're talking about villages of a few dozen to a few hundred thatch-roofed wooden houses with no real firefighting capability. Step 1 is light your torch; step 2 is touch something with your torch. There is no step 3.

Two, the author informs us that the army is spread out. How far is not explicitly stated, but he refers to villages in the plural, and describes a "whole plain covered with horsemen." Large armies could and did spread themselves over quite a distance. The more men in an army, the farther it had to range to feed itself. This was one of the weaknesses of Vegetian warfare - a dispersed army could be attacked and defeated piecemeal. If confronted by a concentrated defending army, the attacker would have to concentrate his own troops, which meant he could no longer feed his army, necessitating battle or retreat. This is exactly what happened at Hastings in 1066, by the way.

Three, we see glimpses of a distinct division of labor. It is not explicitly stated, but we can infer that the foragers and incendiaries are lower-status men. The well-equipped horsemen, with their "helmets glinting in the sun, pennons waving in the breeze," essentially provide overwatch while the foragers and incendiaries go about their work.

The size of a raiding party would vary with the resources of the attacker and the goals of the expedition; a petty lord fighting with his neighbor might lead a force of dozens, a king ten thousand. There's no one size fits all answer available.