In the declaration char a[] = "Visual C++", a is an array of 11 char. So its size is 11 bytes.
In the declaration char *b = "Visual C++", b is a pointer to char. So its size is four bytes (in the C implementation you are using).
In the expression printf("%s", a), a is also an array. However, it is automatically converted to a pointer to the first element of the array. So a pointer to char is passed to printf.
This conversion happens automatically unless an array is the argument of &, sizeof, or _Alignof or is a string literal used to initialize an array of char. Because it happens automatically, people tend to think of array names as pointers. However, they are not.
Incidentally, sizeof is an operator, not a function.
