Warning: function uses 'auto' type specifier without trailing return type

The auto return type "without trailing return type" is a C++14 feature, so I suppose you're compiling C++11.

Your code is OK with C++14, but for C++11, if you want use auto as return type, you need describe the effective return type in this way (caution: pseudocode)

auto funcName (args...) -> returnType

You know that sizeof() returns std::size_t, so your example can be corrected as

constexpr auto MaxEventSize() -> std::size_t
{
    return cexMax(sizeof(int),
           cexMax(sizeof(int),
                    sizeof(int)));
};

or (silly, in this case, but show the use in more complex examples)

constexpr auto MaxEventSize() -> decltype( cexMax(sizeof(int),
                                                  cexMax(sizeof(int),
                                                         sizeof(int))) )
{
    return cexMax(sizeof(int),
           cexMax(sizeof(int),
                    sizeof(int)));
};