implementation of sizeof operator

The sizeof operator is part of the C (and C++) language specification, and is implemented inside the compiler (the front-end). There is no way to implement it with other C constructs (unless you use GCC extensions like typeof) because it can accept either types or expressions as operand, without making any side-effect (e.g. sizeof((i>1)?i:(1/i)) won't crash when i==0 but your macro my_sizeof would crash with a division by zero). See also C coding guidelines, and wikipedia.

You should understand C pointer arithmetic. See e.g. this question. Pointer difference is expressed in elements not bytes.

The result of pointer subtraction is in elements and not in bytes. Thus the first expression evaluates to 1 by definition.

This aside, you really ought to use parentheses in macros:

#define my_sizeof(x) ((&x + 1) - &x)
#define my_sizeof(x) ((char *)(&x + 1) - (char *)&x)

Otherwise attempting to use my_sizeof() in an expression can lead to errors.