The ME Molecular Assembler Chamber is not a specific block but is a combination of blocks that together combine to form a single unit useable as part of an ME Network from the Applied Energistics mod. For those familiar with Railcraft the construction of the most basic MAC is similar to that of the Coke Oven.
To "craft" an ME MAC you need to first build a frame (all edges and corners of a rectangle or square) using ME Assembler Containment Wall. Then fill in the walls, ceiling and floor of the structure with ME Heat Vent. Before closing up the structure entirely you need to completely fill it (inside) with at least ONE ME Pattern Provider and as many ME Crafting CPUs as you would like, which allow your ME network to store patterns for auto-craft recipes placed inside the Pattern Provider.
Once the structure has been properly assembled (frame of Containment Walls, insides filled with pattern providers and/or crafting cpus, and outside faces filled in with Heat Vents) the structure will "complete" and the graphics will change to indicate the MAC has been properly assembled.
As you get more resources to build the different MAC components you can break pieces off and rebuild it with no loss of the stored patterns.
The MAC doesn't need to be a Cube, it can be any Cuboid. The smallest MAC you can build, by definition, is a 3x3x3 cube containing one ME Pattern Provider.
MAC's do not have to have all sides the same length, such as a 4x3x5. This shape is a cuboid, and will work perfectly well.
The largest is not yet known (unless indicated in code somewhere) but MAC containing over 225 CPUs and over 20 Pattern Providers have all worked with no issues. 25x25x25 MAC's consisting of 12,103 CPUs, 64 Pattern Providers, 3,174 Heat Vents and 284 Containment Walls have been made successfully as well, although it causes a large amount of lag.