Both incorrect. It's in 4x4 blocks (16 pixels). DXT only stores two colors per block and interpolates the others, using 2 intermediate colors for DXT1, and either 4 or 6 (can't recall) for DXT2/3/4/5. Edit... made a quick edit of the page. Super busy, working the weekend in fact, but hope to add more later.
One thing to note, at least in my experince when dealing with compression is that Diffuse + alpha = the same amount of texture memory compressed as diffuse, and full color spec compressed. The reason for this is that a standard 24 bit image will take 1/4th the amount of memory compressed, but a 32 bit image with alpha will…