really? how do do change the shape of a block to one that is not in the list?
easy. take a block in an active hot bar slot.
shift + arrow keys / pgup/dn will change the block dimensions
if you activate ALT GR (the key right of the space bar) the top side of the block will turn red. Then you use the same keys to change the dimensions of the surface, giving the block a very unique shape. leave that mode by pressing again ALT GR.
And last but not least... use the ctrl key to hold the block in place and ctrl + arrow keys / pgup/dn to move around the block and maneuver it into the place you want. it can also be used to slide blocks into each other.
And the general shape of the block you can change by right clicking it in your inventory, change shape and you get a whole variety of given shapes.
have fun!