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).
Once a block has been created it's form can usually be changed to one of:
If the block has 5-General block form due to a previous block edit operation, then only these block form options are available:
Depending on the selected block form the full range of block geometry options is available as described under block forms.