the shape of a block must be a single cuboid
in mcgalaxy:
/[lb command] edit [block id] min [min coords]
/[lb command] edit [block id] max [max coords]
the command is /os lb if you're editing the blocks in your level, /lb if you're editing the blocks in the level but it's not yours or /gb if you're editing blocks for the whole server
in singleplayer (in recent versions of cc):
/blockedit [block id] min [min coords]
/blockedit [block id] max [max coords]
the min and max coords are 3 numbers from 0 to 16
for example to make a torch (assuming you set the textures correctly):
/[lb command] edit [block id] min 7 0 7
/[lb command] edit [block id] max 9 10 9
to make a slab:
/[lb command] edit [block id] min 0 0 0
/[lb command] edit [block id] max 16 8 16