One 1x1x1 is 5x5x6 units tall, not including the stud (actual Lego brick that is, assuming Blockland is the same proportion though). Taking an 8x8x8 cube in Blockland, you need 8x8 (40x40) on the bottom and 40 units tall. 40/6 is 6 2/3 (6 bricks 2 plates). Thus, the 8x8x8 cube is actually a cube.
Edit: A plate is 2 units tall. (2x3plates = 1 brick high)
Edit2: So the smalles cube you can make is a 2x2x2 cube. Below that, you cant get 5/6s of a brick.