The sizeof expression returns an Int32 with the size in bytes of a given type. For example:
sizeof(Int32) #=> 4
sizeof(Int64) #=> 8
For Reference types, the size is the same as the size of a pointer:
# On a 64 bits machine
sizeof(Pointer(Int32)) #=> 8
sizeof(String) #=> 8
This is because a Reference's memory is allocated on the heap and a pointer to it is passed around. To get the effective size of a class, use instance_sizeof.
The argument to sizeof is a type and is often combined with typeof:
a = 1
sizeof(typeof(a)) #=> 4