How to define size of class array parameter in source file?

Question

Imagine I have this class.

Foo.hpp:

#pragma once
#include <cstdint>

class Foo {
  static const std::size_t Size;
  std::uint32_t buffer[Size];
};

I want to calculate Size in the source file, so I don't need to include the files needed for the calculation in the header.

Foo.cpp:

#include "Foo.hpp"

#include "Lib1.hpp" // -> #define Size1 1
#include "Lib2.hpp" // -> #define Size2 2

const std::size_t Foo::Size = Size1 + Size2;

Doing so I get the following error:

In file included from src\Foo.cpp:1:
include/Foo.hpp:6:24: error: size of array 'buffer' is not an integral constant-expression
   30 |   std::uint32_t buffer[Size];
      |                        ^~~~

Is it possible to externaly declare a static const class member? If yes, how?