Which Several Standard Block?
'Solid', 'O', and 'U' Block come in a standard length of 24", a height of 8" and will vary in width.
Solid Block comes in widths of 2", 4", 6" up to 12". 
'O' or Core Block comes in 6", 8", 10", & 12" widths and are drilled out 3"- 4" in diameter to allow for the placement of vertical re-bar and cement grout.         
'U' Block come in similar widths and are used to cast reinforced lintels over doors and windows as well as perimeter bond beams.
Jumbo Block are a standard 24" in height,  32" -  48" in length and come in widths of 4", 6", 8" ... up to 18". The larger block widths require a Minicrane to lift and place.
Article Sources:http://www.aerblock.com/block.html