Read
THIS and search for something about packaging add ons, if you don't know how.
If all you want is a plain cube, of any size, you can just do this for the BLB:
WidthNumber LengthNumber HeightNumber
BRICK
HeightNumber is in plates.
For example, a regular 1x1 brick would be:
1 1 3
BRICK
And a 1x1 flat would be:
1 1 1
BRICK