R'Size is a representation attribute used to get the number of bits of an object or type:
- When applied to an object, 'Size yields the actual number of bits allocated to store the object.
- When applied to types, 'Size yields the minimum number of bits required to store an object of that type.
Thus, the 'Size of a type might be lower than the 'Size of its objects. This is especially important to note for variant records, where a particular instance of the record may be much larger than the smallest possible record instance.
The 'Size attribute may also be used to set the minimum size in bits used to store a type.
with Ada.Text_IO; procedure Attributes_Size is package T_IO renames Ada.Text_IO; package I_IO is new Ada.Text_IO.Integer_IO (Integer); A_Boolean : constant Boolean := True; begin T_IO.Put ("Size of Boolean type = "); -- An enumeration with I_IO.Put (Boolean'Size); -- 2 values fits into T_IO.New_Line; -- 1 bit. T_IO.Put ("Size of Boolean Object = "); -- it is more efficient I_IO.Put (A_Boolean'Size); -- to store a boolean T_IO.New_Line; -- as an entire byte end Attributes_Size;
The output with GNAT 3.4.1 will be:
Size of Boolean type = 1 Size of Boolean Object = 8
Try it yourself and see how your compiler does.
Incorrect usages 
A common Ada programming mistake is to assume that specifying 'Size for a type T forces the compiler to allocate exactly this number of bits for objects of this type. This is not true. The specified T'Size will force the compiler to use this size for components in packed arrays and records and in Unchecked_Conversion, but the compiler is still free to allocate more bits for stand-alone objects.
Use 'Size on the object itself to force the object to the specified value.
See also 
- Ada Programming
- Ada Programming/Attributes
- Ada Programming/Attributes/'Component_Size
- Ada Programming/Attributes/'Max_Size_In_Storage_Elements
- Ada Programming/Attributes/'Value_Size (implementation defined)
- Ada Programming/Attributes/'Object_Size (implementation defined)
Ada 83 Reference Manual 
Ada 95 Reference Manual 
- 13.3: Operational and Representation Attributes (Annotated)
- Annex K: Language-Defined Attributes (Annotated)