It depends on what load is on the wall.
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If it is an exterior shed type storage room you can go as deep and wide as you want. If it is part of a home and attached it should be 12 inches wide and deep enough to get below the frost or freeze depth in your area. If freezing ground is not a problem than 12 inches deep should be fine.
Traditional Concrete Masonry Unit (CMU) Blocks have 2 rectangular voids through the block. These voids are called "Cells". The face shell thickness is the thickness of the block material from the cell to the front of the block. For example, a typical block of dimensions 8"high by 8"deep by 16" long probably has a face shell thickness of 1.25".
If you put £20 notes into stacks three notes wide, by four notes deep, then you'd have a block of bank notes over 45cm high. There are photos of a simulated block of one million pounds on this page - [See related link below for the photos]
The opposite of "deep", as in "deep water", is "shallow". If you mean "deep" in a metaphorical sense, such as a "deep thought", the opposite might also be "superficial".
"Shallow" is the opposite of "deep".