Best for retina is not full 2880x1800... Change the settings to the different level of scale and run
#include <SFML/Graphics.hpp>
#include <iostream>
std::ostream& operator<<(std::ostream& out, sf::VideoMode vm) {
return out << "{" << vm.width << ", " << vm.height << "}";
}
int main(int, char const**)
{
std::cout << "Desktop: " << sf::VideoMode::getDesktopMode() << std::endl;
return 0;
}
and see what happens. You'll see that SFML reflect the user's settings pretty well.