Now create a PCloud-Particle system. I used the dimensions 20m x 20m x 20m. Leave most of the settings to default values, except the following: In the basic parameters-rollout change "Viewport display" to mesh, so you can see what it actually will look like and if you have a fast computer you might change "Percentage of particles" to 100% (this only has an effect on viewport display, when rendering it will always be 100%).
In the particle-generation-rollout change "rate" to something much higher, like 6000 or even more. Change particle type to "instanced geometry" and click "pick object". Pick the cube. Voila! Now you have 6000 (or even more) cubes. The size isn't right yet, and i have to admit that i have no idea why, but i had to change the size-parameter to 0.02m (particle generation-rollout). Looks good now, and looks even better after changing the value for size-variation to something like 50%. Now you have many cubes with varying dimensions.
Some might wonder why i chose instanced geometry of a cube instead of using standard geometry>cube from the particle-type-rollout. The reason is that the standard cubes won't have the material-id's set correctly. What are material id's? Material id's are assigned to each polygon and determine which submaterial from the Multi/Subobject-Material will be assigned. Material id 1 will assign the material that is in the first slot, Mat id 2 will assign the second one and so on. Our instanced cube has a different material id for each face, so we have all 6 submaterials on the cube. We want that for the instanced cubes of the Particle cloud too, so we need to do use instanced geometry and then do one more thing: in "material mapping and source" at the bottom of our "particle-type"-rollout click "get material from" and make sure that the "instanced geometry"-radiobutton is set (pics 19 and 20). In a shaded perspective viewport you should see a result similar to pic 17.
The geometry is almost finished.