Can't a class have static constexpr member instances of itself?

This code is giving me incomplete type error. What is the problem? Isn't allowed for a class to have static member instances of itself? Is there a way to achieve the same result?

struct Size
{
    const unsigned int width;
    const unsigned int height;

    static constexpr Size big = { 480, 240 };

    static constexpr Size small = { 210, 170 };

private:

    Size( ) = default;
};

Is there a way to achieve the same result?

By "the same result", do you specifically intend the constexpr-ness of Size::big and Size::small? In that case maybe this would be close enough:

struct Size
{
    const unsigned int width = 0;
    const unsigned int height = 0;

    static constexpr Size big() {
        return Size { 480, 240 };
    }

    static constexpr Size small() {
        return Size { 210, 170 };
    }

private:

    constexpr Size() = default;
    constexpr Size(int w, int h )
    : width(w),height(h){}
};

static_assert(Size::big().width == 480,"");
static_assert(Size::small().height == 170,"");