Block tab on Model Options dialog under Block Geometry header.
You can change the Block wall thickness of the current block by entering the new thickness here. The thickness must be in the range 0.01m - 2.0m (0.39 - 78 inches).
You can choose to switch on/off display of external walls for each block. Generally for standard extruded blocks it is best to show external walls to help reference location within the block with respect to existing openings etc. For roof blocks and other non-plan extrusion blocks the external walls can obscure the internal space and so the default for these block types is to hide the external walls. You can override the defaults using this option. It can be useful to switch off external wall display when drawing partitions in roof blocks.
Once a block has been created it's form can usually be changed to one of:
For more on block forms see help on creating blocks.