Calling a base member in constructor in multiple inheritance in C++

Suppose I have these two classes

class base_size
{
public:
   int size()
   { return 5; }
};

class base_implement
{
public:
   base_implement(int s) : _vec(s)
   {
      cout << "size : " << _vec.size() << endl;
   }
private:
   vector<float> _vec;
};

If I were to inherit from both of these would it be alright to call one of these classes member function in the other's constructor? For example

class derived : 
   public base_implement,
   public base_size
{
public:
   derived() : base_size(), base_implement(size())
   {
      // Is this OK? 
      // If derived is not yet constructed can I access this->size() ?
      // Works in VC++. Not sure about other compilers.
   }
};