Вы можете использовать Boost Integer для выбора правильных случаев.

• http://www.boost.org/doc/libs/1_54_0/libs/integer/doc/html/boost_integer/integer.html#boost_integer.integer.sized

игнорируя на мгновение случаи на нецелочисленных типах элементов, вот быстрый тест предлагаемых случаев (GCC не имеет int128_t как оно выглядит):

жить на Coliru

#include <boost/mpl/vector.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/max_element.hpp>
#include <boost/integer.hpp>
#include <limits>

using namespace boost;

namespace best_fit_
{
    // wrappers around Boost Integer http://www.boost.org/doc/libs/1_54_0/libs/integer/doc/html/boost_integer/integer.html#boost_integer.integer.sized
    template <bool is_signed, int bin_digits> struct select_int;

    template <int bin_digits> struct select_int<true, bin_digits> {
        using type = typename boost::int_t<bin_digits + 1>::least;
    };

    template <int bin_digits> struct select_int<false, bin_digits> {
        using type = typename boost::uint_t<bin_digits>::least;
    };

    // query helper
    struct digits {
        template <typename I> using apply = mpl::int_<std::numeric_limits<I>::digits>;
    };
}

template <typename... I>
struct best_common_integral
{
    private:
        using Ints = mpl::vector<I...>;
        using Bits = typename mpl::transform<Ints, best_fit_::digits>::type;

        template <typename J>
            struct is_signed { static constexpr bool value = std::numeric_limits<J>::is_signed; };

        using max  = typename mpl::deref<typename mpl::max_element<Bits>::type>::type;

        // sigh, there is no `mpl::any`, AFAICT
        using sign = typename mpl::fold<
                    Ints, 
                    mpl::bool_<false>, 
                    mpl::if_<is_signed<mpl::_2>, mpl::bool_<true>, mpl::_1>
                >::type;
    public:
        using type = typename best_fit_::select_int<sign::value, max::value>::type;
};

#include <typeinfo>
#include <iostream>
#include <cassert>

int main()
{
    using case1 = best_common_integral<long, unsigned long, float, double, int>;
    using case2 = best_common_integral<unsigned int, unsigned long>;
    using case3 = best_common_integral<signed int, signed long>;
    using case4 = best_common_integral<signed int, unsigned int>;
    using case5 = best_common_integral<signed int, unsigned long>;

    //assert(typeid(case1::type) == typeid(double));
    assert(typeid(case2::type) == typeid(unsigned long));
    assert(typeid(case3::type) == typeid(signed long));
    assert(typeid(case4::type) == typeid(signed long));
    //assert(typeid(case5::type) == typeid(int128_t (maybe)));
}

Примечание: как-то я застрял в моей голове, что вам нужен C++03 для этого. Это можно упростить для C++11. Это также не выбирает наименьший размер.

нет ничего стандартного для этого, насколько мне известно, но это можно сделать: http://coliru.stacked-crooked.com/view?id=c6aa42345f91ab51d745d56573b15a04-4f34a5fd633ef9f45cb08f8e23efae0a

сначала структуры "мыслителя".

template<bool isfloat, bool negative> struct best_numeric_type 
{typedef long double type;};
template<> struct best_numeric_type<false, true> 
{typedef long long type;};
template<> struct best_numeric_type<false, false> 
{typedef unsigned long long type;};

затем база случаи:

template<class T> struct best_common_numeric_type1 {
    static const bool isfloat=false;
    static const bool negative=false;
    typedef typename best_numeric_type<isfloat, negative>::type type;
};

template<> struct best_common_numeric_type1<char> {
    static const bool isfloat=false;
    static const bool negative=true;
    typedef typename best_numeric_type<isfloat, negative>::type type;
};//copy-paste for signed char, short, int, long, and long long.

template<> struct best_common_numeric_type1<float> {
    static const bool isfloat=true;
    static const bool negative=false;
    typedef typename best_numeric_type<isfloat, negative>::type type;
};//copy-paste for double and long double.

затем Столяров:

template<class First, class Second>
struct best_common_numeric_type2 {
    static const bool isfloat = best_common_numeric_type1<First>::isfloat |  best_common_numeric_type1<Second>::isfloat;
    static const bool negative = best_common_numeric_type1<First>::negative |  best_common_numeric_type1<Second>::negative;
    typedef typename best_numeric_type<isfloat, negative>::type type;
};
template<class First, class Second, class Third>
struct best_common_numeric_type3 {
    static const bool isfloat = best_common_numeric_type2<First, Second>::isfloat |  best_common_numeric_type1<Third>::isfloat;
    static const bool negative = best_common_numeric_type2<First, Second>::negative |  best_common_numeric_type1<Third>::negative;
    typedef typename best_numeric_type<isfloat, negative>::type type;
};
template<class First, class Second, class Third, class Fourth>
struct best_common_numeric_type4 {
    static const bool isfloat = best_common_numeric_type3<First, Second, Third>::isfloat |  best_common_numeric_type1<Fourth>::isfloat;
    static const bool negative = best_common_numeric_type3<First, Second, Third>::negative |  best_common_numeric_type1<Fourth>::negative;
    typedef typename best_numeric_type<isfloat, negative>::type type;
};
template<class First, class Second, class Third, class Fourth, class Fifth>
struct best_common_numeric_type5 {
    static const bool isfloat = best_common_numeric_type4<First, Second, Third, Fourth>::isfloat |  best_common_numeric_type1<Fifth>::isfloat;
    static const bool negative = best_common_numeric_type4<First, Second, Third, Fourth>::negative |  best_common_numeric_type1<Fifth>::negative;
    typedef typename best_numeric_type<isfloat, negative>::type type;
};

и, наконец, тест:

#include <typeinfo>
#include <iostream>       
void printer(long double) {std::cout << "long double\n";}
void printer(unsigned long long) {std::cout << "ull\n";}
void printer(long long) {std::cout << "ll\n";}
void printer(...) {std::cout << "else\n";}

int main() {
    printer(best_common_numeric_type5<long, unsigned long, float, double, int>::type());
    printer(best_common_numeric_type2<unsigned int, unsigned long>::type());
    printer(best_common_numeric_type2<signed int, signed long>::type());
    printer(best_common_numeric_type2<signed int, unsigned int>::type());
    printer(best_common_numeric_type2<signed int, unsigned long>::type());
    printer(best_common_numeric_type2<float, char>::type());
}

результаты:

long double
ull
ll
ll
ll
long double